Consider allowing fish fields to be included in filtering

[thetron]

This would need a neat, simple syntax - but might help to enhance functionality of the filtering a little. Filter groups for field filtering could be assigned at filter-time.

.query({
  tags: {...},
  filters: {...},
  filter_fields: {
    field_slugged_something: { 
      value: Fishpond.blank_field  // searches for when the field is blank, we could define a bunch of methods for generic values, perhaps?
      group: 0 // optional group param, should this need to be 'and-ed' against another filterset
    },
  } 
});