Added ability to specify QueryLoader and Query class

[xlevus]

I need the ability to customize the generation of the Queries class to add additional functionality.

This should allow me to do that.

[nackjicholson]

@xlevus Thank you so much for your contributions! This is a good non-breaking change to the API to allow you to plugin whatever extensions you need for your Queries class.

I definitely never intended for anyone to do that, but it's cool. I am curious about these additional things you're doing to your Queries class though. What are your extra needs/requirements?

[xlevus]

The main use-case for this was to add a small bit of functionality to support table sharding.

Given you can't (easily) parameterize the table name, our Queries subclass just piped the SQL string through "".format().

[nackjicholson]

I just opened another issue #21 that links to another couple of similar sounding issues in honza/anosql. Would a feature like that be of help?

Some feature that could somehow use psycopg2.sql.Identifier underneath.