Add SELECT DISTINCT feature to queryset

[pmdevita]

Is your feature request related to a problem? Please describe.
Currently, there is no way to perform a SELECT DISTINCT query in Ormar.

Describe the solution you'd like
An addition to the queryset features that allows you to perform a SELECT DISTINCT.

Describe alternatives you've considered
At the moment, you can either execute the SQL directly or do the extra processing in Python, both of which are less than ideal.

Additional context
We could add a .distinct() function to queryset to allow for chaining it similar to Django.