ExplainaBoard V0.11

[neubig]

This is a major refactoring of ExplainaBoard to:

1. Introduce the concept of "analysis levels", where analysis can be performed over examples, spans, tokens, etc. separately. In order to implement this, the code is refactored to first generate "analysis cases" for each of these levels, then perform analysis over the analysis cases.
2. Separate definition of features from definition of analyses. This allows for multi-feature analyses (like confusion matrices, etc.). Features can also be defined directly through feature functions instead of relying on separately implemented functions.