Add support for extensionless files, other improvements

* If a relative file has no extension, a HEAD request is performed to see if it exists, and if not, a `.js` is added to the url. Fixes GH-2.
* Support cases like `require('foo/bar')`, going to the npm module `foo`
* Ensure valid package name. Fixes the case `require(someVariable + 'string')` by skipping over it, instead of erroring out.

The fix for GH-2 isn't the cleanest, but it's the best solution I could think of. Love the extension! :smile: