Support more complex join conditions for relationships

[danielrearden]

The API currently only supports joining two models using a single foreign key. This can be limiting if the relationship needs to support additional join conditions. For example, if using a closure table and wanting to join based on a particular depth. Or applying arbitrary filters based on the target table's column values (like having an activeCustomers field on store). The current workaround is to utilize inline views to create separate models for these use-cases, but this is not ideal. This might warrant a bigger change to the existing API (moving away from the "from" and "to" syntax entirely).