Modify extension handling to allow for .coffee.md

Move filename processing to a `parseFileName` function in
helpers.coffee.

Map `.coffee.md` as a Literate CoffeeScript extension.

Also, make .litcoffee and .coffee.md files executable without their file
extension - eg. `coffee test` would work for a file called
`test.litcoffee`.
Merging in .coffee.md support alongside .litcoffee ... I think we'll keep .litcoffee as the canonical, however.