Apps can hook in their own locator.

Adds #7.
This instantiates a new ActiveRecordFinder.new for each call. We only need once instance of it. For example, could memoize like
```ruby
@locators.fetch(gid.app) { default_locator }
```
```ruby
mattr_accessor :default_locator
self.default_locator = ActiveRecordFinder.new
```