Markdown: render pages under /+doc/

Parse and render any file ending with ".md" as markdown using the
pegdown processor.  This allows a repository to be used as a
documentation source.

If "navbar.md" is found in the top level directory it is rendered as a
horizontal bar across the top of the page.  A navbar.md is an outline:

  * [Home](/index.md)
  * [APIs](/api/index.md)
  * [Source](/src/main/java/index.md)

Any links starting with "/" and ending in ".md" are resolved as
relative to the top of the Git repository.

As an extension to Markdown the special block [TOC] is replaced with
a table of contents from the headers of the text.

Markdown support can be optionally disabled by setting the config
variable markdown.render to false.

Change-Id: Ic111628c59cfadfdca37bf0cc637ee8a14d54c37
11 files changed