First pass at a bower extension

This is a first draft of the bower extension. Works as described in the
issue.

Possible problems:

1. This will try to download a `bower.json` by default. This might not be
desirable to everyone.
2. The user might want to import a js file other than `main` which this
won't allow. Probably solution is to check if a `path` is already set
for the module and if so skip trying to load from bower.