Gen shortcut for `COUNT(DISTINCT column_name)`

[renom]

There's Count method that is generated for a table:

Count(ctx context.Context, exec boil.ContextExecutor) (int64, error) // performs COUNT(*)

It'd be useful to have CountDistinct method:

CountDistinct(ctx context.Context, exec boil.ContextExecutor, column string) (int64, error) // performs COUNT(DISTINCT column)

[stephenafamo]

If you send in a PR, I'll be happy to review.