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.