WIP: Aggregates

I still need to add tests, so this code isn't quite ready to merge, but all the functionality should be there...
Seems like aggregates are sophisticated enough to merit their own query builder (standalone that would ultimately 'compile' down to the string)? I suppose even there was one, we'd still want a version that takes the raw String, so perhaps we can wait on building out an aggregation query builder until we get some user feedback on it. (thinking out loud).