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
`:erlang.now` is guaranteed to be unique, do we need something more?