Add multi tenancy capabilities with Ecto meta prefix

[dreamingechoes]

Sometimes it's required to work on a multi tenancy system, and be able to create new attributes for the records/models on different schemas/databases. Attributex doesn't allow this at this moment.

Expected behavior

Be able to use attributex in a multi tenant system.

Current behavior

Now it's not possible.

Possible solution

Leverage on the Ecto.Query prefix in order to switch between different schemas/databases. It's necessary to let the user specify this value from the options of the functions.