Markup README a bit more.
Modify layouts.core/Parse->string to throw helpful exceptions.
Split some of page layout into base implemention.
Define markdown-to-html function in impl.markdown.
Change name of parser fn in impl.clj.
Document parser.core a lot. Change expected behaviour of parsers.

They should take a file and return a thunk or delay.
Modify html-parser to return a delay.

The delay should return a sequential collection of files.
Modify copy impl to work with new parsers format.

Returns a delay which returns a sequential collection of created files.
Use new parsers.core functions in once and middleware.