Create index helpers for models

[justinperkins]

Since the partitioned tables will/can get created outside the scope of a migration, it makes sense to add those helper methods (add_index) to the model helpers.

This would compliment the existing helpers to manage creating partitioned tables, like "create_partition".

[rkrage]

Sorry, I've been crazy busy lately. I'm currently working on changing some APIs around and making things more consistent. I'll make sure this is included

[justinperkins]

I'm working on abstracting all of my model-based needs into a concern, was going to propose it as a PR once I worked everything out. For the time being, this works perfectly fine in the model ...

self.connection.add_index my_partition_name, [...], name: "..."

[rkrage]

Yeah I think it would definitely be a good idea to break up some of the model functionality into discrete concerns. I was playing around with some pretty weird patterns when I initially wrote this thing too. There's a fair amount of cleanup that I want to do before releasing a 1.0 version