Markdown: add minimal support for AST to HTML

Includes extension support to walk the AST and produce a table
of contents from the contained HeaderNodes.

Rendered HTML is blessed as SanitizedContent for Soy to include
inside of templates without further escaping.

Change-Id: Ib53e091617b39e22a230da9ec8a4aa39c3bf5c26

gitiles-servlet/BUCK

diff --git a/gitiles-servlet/BUCK b/gitiles-servlet/BUCK
index 9fdacae..9e7a1e8 100644
--- a/gitiles-servlet/BUCK
+++ b/gitiles-servlet/BUCK
@@ -2,10 +2,12 @@
 RSRC = glob(['src/main/resources/**/*'])
 DEPS = [
   '//lib:commons-lang',
+  '//lib:grappa',
   '//lib:gson',
   '//lib:guava',
   '//lib:joda-time',
   '//lib:jsr305',
+  '//lib:pegdown',
   '//lib:prettify',
   '//lib/jgit:jgit',
   '//lib/jgit:jgit-servlet',