Support multiple databases

[uselessscat]

Allows to use multiple databases with different migrations.

Example layout

-- src
  |-- app
  |  |-- models
  |  |-- routers
  |  |-- ...etc
  |-- tests
  |-- databases
     |-- migrations
        |-- databaseA
        |  |-- ...alembic files
        |-- databasB
           |-- ...alembic files