Adds documentation for the documentation macros.
Merge pull request #8 from MichaelHatherly/mh/document-documentation-macros

Adds documentation for the documentation macros.
Stub html writemimes.
WIP
Merge branch 'mh/html-rendering' of git://github.com/MichaelHatherly/Docile.jl into mh/html-rendering
Rough html rendering. Requires local mods to Markdown.jl