Import the markdown spec

The bulk of the effort was done in:
https://github.com/iainmcgin/scala-ref-markdown/

I just ported it to redcarpet 3.1, added a jekyll build to run on Travis,
and fixed some long-standing spec bugs.

As described in spec/README.md, for now we're only generating
an html page per each chapter with a rudimentary toc index.
We'll focus on presenting this more nicely first, and then
look into a pdf / single-page html version.

PS: I'm submitting this PR to the master-tmp branch, so that the bot will ignore it (since there's no milestone that targets it)