Add multi tenancy capabilities with Ecto meta prefix

[dreamingechoes]

Sometimes it's required to work on a multi tenancy system, and be able to
keep track of the versions of the records/models on different schemas/databases. PaperTrail
doesn't allow this at this moment.

Expected behavior

Be able to specify the tenant where the versions entries generated by the user's activity will be stored.

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.