Implemented expand_by_class. Closes #362.

 Optional argument separators requires changes Classify and FindByClass (not currently implemented for find_by_class either), will be implemented later. Will require implementing Py_find_by_class manually.
This is a subtle change worthy of one or more new entries in the [TestFindByClass test matrix](https://github.com/limetext/lime/blob/9b0c8717cc00fbc5cfd44982bd47b6c0a3d67420/backend/view_test.go#L423) also.