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
This needs to be fixed to not depend on file contents. I wanted to use `:random`, but the function call may run from different processes (e.g. during testing) and the random state is local to each processes.

Any suggestions?