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
Please don't use `:erlang.now` for anything, ever.

Use: `:erlang.phash2(:crypto.rand_bytes(8))`