Allow direct go generate usage (no CLI)

[bojanz]

Right now I need to modify my Makefile to install broccoli in $PATH to be able to use it from go generate. It would be simpler if I could instead invoke my own gen.go which just calls the broccoli API, like vfsgen allows. From what I can see, main.go is already a very thin wrapper, all that's needed is to export Generator options and parsePackage() / generate() functions. Is that something you'd be open to? If yes, I could send in a PR.