[FEATURE REQUEST] Option for label ranking in outputs

[AndriyMulyar]

Description
A CRF produces label probability outputs. Currently, we are simply using the highest probability label as the predicted entity label. It would be useful to allow for an option to output a label ranking for a given token.

Uses
Multi-label prediction, hierarchical label prediction. Capable of handling nested entities.

Where to get started
A contributor would simply have to interface the correct attributes set by the sklearn-crfsuite wrapper into the Annotation class. Some discussion would have to be had to insure compatibility with other parts of the package.