If search param is not present as field in a model

There are going to be cases where someone wants to search using a parameter that will require additional logic than just a simple lookup.

Case in point: a through-type association.

`phobos.js` should first look to see if a field exists, and if it doesn't but it's whitelisted in `searchableBy`, then it should look for a function to run in the model's static methods.
