New Config: add min_priority to .credo.exs

[kelvinst]

What do you want Credo to do?

I know we have the --min-priority flag on CLI, but it would be really nice to be able to set the default min priority for the project, so we don't have to set it on CLI every time.

Also, it would be awesome to document somewhere which one is the default min priority level.

Which existing behaviour would change?

People would be able to set min_priority option on .credo.exs and that would be used when running mix credo without any config.