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
Thanks, great post. One thing I don't agree with is that a True Random Number Generator has to produce each value with equal probability. That only holds for a TRNG with uniform distribution.

:innocent: