Add a post-bundle callback option

I've been doing some work with caching transforms and I found that I needed a callback for when browserify has finished bundling to close connections to leveldb.

In this pull request I've added a `post` option to the configuration, which takes a function to be called when the bundling process is complete.

I've also included a test, which passes. Though I had to turn off the external test, for some reason it can't find a vendor/alias.

Let me know if this needs more work.