Add ability to include the file extension when requiring modules.

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