Add an add-on generator

[vinistock]

Add-ons have a bit of boilerplate code that is tedious to setup. We should provide some sort of generator that can both add the add-on structure to an existing gem or create a brand new add-on gem (essentially, invoke bundle gem and then make some modifications on top of it).

We could provide some example code as part of it, displaying indexing enhancements, listener contributions and so on.