Add metrics and tools for model comparison

[x-tabdeveloping]

As per suggested by @Jemoka:

Adding utilities for comparing topic models would make the library stand out from the rest of the crowd, and would make a more convincing point for adoption by most people.

I think the following would make quite a bit of sense:

1. Embedding-based coherence on:
   1. Words
   2. Documents
   3. Images
2. Diversity metrics:
   1. Embedding-based
   2. Lexical

It would also make a lot of sense to add the option for significance tests, since most of these metrics are aggregated.

[KennethEnevoldsen]

@x-tabdeveloping, can you outline what you think is the desirable user interface here? Then we can just discuss from there?

(I could imagine that people might want to compare models visually down the line as well, but I don't think this is now)