Serve Markdown project documentation as HTML pages

This plugin serves Markdown project documentation as HTML pages.

If projects contain documentation as Markdown files, the plugin
automatically serves the generated HTML under
`/xdocs/project/<project-name>/<file-name>`. The project name and
the file name must be URL encoded.

The file is served from the branch/commit to which `HEAD` points unless
a revision is specified in the URL as
`/xdocs/project/<project-name>/rev/<rev>/<file-name>`.

E.g.:
  /xdocs/project/external%2Fopenssl/rev/stable-1.3/docs%2Ffaq.md

`rev` can be any ref or commit that is visible to the calling user.

If the file name is omitted the plugin serves the `README.md` from the
project if available.

All HTML blocks as well as inline HTML tags are suppressed. Both will
be accepted in the input but not be contained in the output.

Change-Id: I7872722c67872e1be915b54d82e56bad8cab2275
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>