Support playhouse.ManyToManyField asynchronously

[marijngiesen]

Currently, using a ManyToManyField from the playhouse package triggers a sync_query. It would be awesome if this could be handled asynchronously.

[rudyryk]

This is definitely overlaps with #15

Sync interface is here by peewee's design, so we can't handle it without breaking into peewee or without providing incompatible interface. And one of goals of peewee-async is not to break compatibility with peewee itself.

For now you should construct such joins manually.