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
I understand that we don't have a release process for ExDoc yet. However, the only strict requirement for it is to be able to run with a given Elixir version.

Therefore I think we should be able to host ExDoc releases and update them at will (without changing the URL) as long as each update works with the corresponding Elixir version.