Pandoc

This change does the following things:

1. Provides an alternative to the `sundown` library for converting markdown to HTML.
2. Lays down further groundwork for supporting custom formatters (will add ability to configure used markdown processor in the next PR).
3. Allows ex_doc to be used from a locally installed archive.

/cc @pminten @ericmj
We can also copy the small heuristic in plug/dynamo where we generate
something, check it doesn't exist and go with it. If it exists, we try
again.

-- 


*José Valimwww.plataformatec.com.br
<http://www.plataformatec.com.br/>Founder and Lead Developer*