Add support for word embedding like features which are list of floats

The current API doesn’t support adding features which are list of floats e.g. Word Embeddings. The current approach to add these features is to do something like {"f0": 1.5, "f1": 1.6, "f2": -1.4} for 3 dimensional embedding features, which adds extra burden on the user’s part.

I propose a wrapper feature which will allow users to pass the word embedding list as the value of the dictionary. E.g. {"f": FloatFeatures([1.5, 1.6, -1.4])}, internally this will convert the float features into a representation consistent with the CRFSuite ItemSequence and having a consistent naming convention like "f:0", "f:1", "f:2".

[EmilStenstrom]

Using word embeddings improve accuracy a lot. Having a supported way to include them in python-crfsuite would be wonderful.

[napsternxg]

The approach I suggested is utilized in this tool I have built.

https://github.com/napsternxg/TwitterNER