Fix incompatibility with Browserify CLI transforms

This fixes issue #1. It adds compatibility with the Browserify CLI's `-t` option. The required API change is that the main exported function now just takes a file as an argument and returns a stream. Custom extensions can still be configured with a `.configure()` function. Otherwise the API is the same. I also took care of documenting the change. The patch is based on [hbsfy](https://github.com/epeli/node-hbsfy) as I suggested in the issue.