Add ability to include the file extension when requiring modules.

[rehno-lindeque]

For example, instead of

require "foo/bar"

it is possible to write

require "foo/bar.js"

or

require "foo/bar.coffee"

I believe this behavior is compatible with node.js. (I ran into a problem with stitch where I had a module called adt.js. Using node require pulls in the package called adt.js, but with stitch I instead wanted to require the single file called adt.js that I stitched in).

Cheers