Add ability to collapse and rename classes from configuration file

I've added a `class_map` setting for the config files that lets you collapse multiple labels into a single label by specifying a dictionary.

For example, if you wanted to collapse the classes "beagle" and "dachsund" into a "dog" class, you would specify the following for `class_map`:
```python
{'dog': ['beagle', 'dachsund']}
```
Any classes not included in the dictionary will be left untouched.