Generator registration

Would just like eyes on two things:

1. Is the addition to the README clear and helpful?
2. Does the way described in the README sound like a pleasant way to work with the GeneratorManager?
We could change the signature to be `register(name, &block)`, or `register(name, generator, &block)` and then do `block_given? ? &block : generator`. It seems not very Ruby-ish to force the proc to be an argument instead of a block. Everything else should continue to work as-is.