Support for cheatsheets as manpages

@arthurnn:

SUPER experimental, possibly junky screwing around, support for writing cheatsheets in markdown, then being able to load them in manual pages.

disadvantages: 
 - we end up cashing the roff formats locally on the first time a user runs a cheatsheet
 - stack seems a bit dirtier than before
 - ugly exec to call man

advantages:
 - markdown is easier to write than yaml
 - use kramdown, so we'd be able to build cheatsheets into latex, which opens up a whole new world of cheatsheets
 - support man pages, which means we can generate all kinds of interesting and complex cheatsheet formats to review in console
 
I only implemented help.md for now, and there are probably bugs involving the github adapter, etc etc. this is a reference implementation for discussion :)