Make all Markdown extensions configurable

Allow site admins to selectively enable or disable Markdown
extensions.  This can help a site avoid creating documentation
that isn't compatible with other CommonMark parsers.

Change-Id: I0466b03ef213a398d79f943af2ddf95a7e0853e7

Documentation/config.md

diff --git a/Documentation/config.md b/Documentation/config.md
index 65feff2..27db57a 100644
--- a/Documentation/config.md
+++ b/Documentation/config.md
@@ -52,8 +52,51 @@
   imageLimit = 256K
 ```
 
+### Extensions
+
+The following extensions can be enabled/disabled in the markdown
+section:
+
+* `githubFlavor`: enable extensions that mirror GitHub Flavor
+  Markdown behavior.  Default is true.
+
+* `autolink`: automatically convert plain URLs and email
+  addresses into links. Default follows `gihubFlavor`.
+
+* `blocknote`: Gitiles style note/promo/aside blocks to raise
+  awareness to important content. Default false.
+
+* `ghthematicbreak`: accept `--` for `<hr>`, like GitHub Flavor
+  Markdown.  Default follows `githubFlavor`.
+
+* `multicolumn`: Gitiles extension to layout content in a 12 cell
+   grid, delinated by section headers. Default false.
+
+* `namedanchor`: Gitiles extension to extract named anchors using
+  `#{id}` syntax. Default false.
+
+* `safehtml`: Gitiles extension to accept very limited HTML; for
+   security reasons all other HTML is dropped regardless of this
+   setting.  Default follows `githubFlavor`.
+
+* `smartquote`: Gitiles extension to convert single and double quote
+  ASCII characters to Unicode smart quotes when in prose.  Default
+  false.
+
+* `strikethrough`: strikethrough text with GitHub Flavor Markdown
+  style `~~`.  Default follows `githubFlavor`.
+
+* `tables`: format tables with GitHub Flavor Markdown.  Default
+  follows `githubFlavor`.
+
+* `toc`: Gitiles extension to replace `[TOC]` in a paragraph by itself
+  with a server-side generated table of contents extracted from section
+  headers.  Default true.
+
 ### IFrames
 
+IFrame support requires `markdown.safehtml` to be true.
+
 IFrame source URLs can be whitelisted by providing a list of allowed
 URLs. URLs ending with a `/` are treated as prefixes, allowing any source
 URL beginning with that prefix.

Documentation/markdown.md

diff --git a/Documentation/markdown.md b/Documentation/markdown.md
index 958fed9..23e3bae 100644
--- a/Documentation/markdown.md
+++ b/Documentation/markdown.md
@@ -150,6 +150,8 @@
 
 ### Tables
 
+Requires `markdown.tables` to be true (default).
+
 Simple tables are supported with column alignment.  The first line is
 the header row and subsequent lines are data rows:
 
@@ -199,6 +201,8 @@
 
 ### Strikethrough
 
+Requires `markdown.strikethrough` to be true (default).
+
 Text can be ~~struck out~~ within a paragraph:
 
 ```
@@ -313,9 +317,10 @@
 
 ### Horizontal rules
 
-A horizontal rule can be inserted using GitHub style `--` surrounded
-by blank lines.  Alternatively repeating `-` or `*` and space on a
-line will also create a horizontal rule:
+If `markdown.ghthematicbreak` is true, a horizontal rule can be
+inserted using GitHub style `--` surrounded by blank lines.
+Alternatively repeating `-` or `*` and space on a line will also
+create a horizontal rule:
 
 ```
 ---
@@ -380,7 +385,7 @@
 ### Named anchors
 
 Explicit anchors can be inserted anywhere in the document using
-`<a name="tag"></a>` or `{#tag}`.
+`<a name="tag"></a>`, or `{#tag}` if `markdown.namedanchor` is true.
 
 Implicit anchors are automatically created for each
 [heading](#Headings).  For example `## Section 1` will have
@@ -460,9 +465,9 @@
 by the parser with no warnings, and no output from that section of the
 document.
 
-There are small exceptions for `<br>`, `<hr>`, `<a name>` and
-`<iframe>` elements, see [named anchor](#Named-anchors) and
-[HTML IFrame](#HTML-IFrame).
+If `markdown.safehtml` is true there are small exceptions for `<br>`,
+`<hr>`, `<a name>` and `<iframe>` elements, see [named anchor](#Named-anchors)
+and [HTML IFrame](#HTML-IFrame).
 
 ## Markdown extensions
 
@@ -471,6 +476,8 @@
 
 ### Table of contents
 
+Requires `markdown.toc` to be true.
+
 Place `[TOC]` surrounded by blank lines to insert a generated
 table of contents extracted from the H1, H2, and H3 headers
 used within the document:
@@ -496,6 +503,8 @@
 
 ### Notification, aside, promotion blocks
 
+Requires `markdown.blocknote` to be true.
+
 Similar to fenced code blocks these blocks start and end with `***`,
 are surrounded by blank lines, and include the type of block on the
 opening line.
@@ -538,6 +547,8 @@
 
 ### Column layout
 
+Requires `markdown.multicolumn` to be true.
+
 Gitiles markdown includes support for up to 12 columns of text across
 the width of the page.  By default space is divided equally between
 the columns.
@@ -608,6 +619,8 @@
 
 ### HTML IFrame
 
+Requires `markdown.safehtml` to be true (default).
+
 Although HTML is stripped the parser has special support for a limited
 subset of `<iframe>` elements: