text: Title: Read/Write database selection
  Body: **Is your feature request related to a problem? Please describe.**
  I need to implement read and write replicas of my database in my app, I know I can pass the db to the model query directly using use_db.
  
  **Describe the solution you'd like**
   It would be great if when creating my OrmarConfig I could define a read only database that will get used for all read operations and the default database will be used for write, what would be even better. 
  
  **Describe alternatives you've considered**
  using the use_db function but seems a bit messy, this is a config that a lot of other frameworks provide so would be good to consider for a future release.
  
  **Additional context**
  Not that I can think of.
  