Print n-gram vectors

[englhardt]

Extension of print-ngrams to extract multiple n-gram vectors at once. Work around until a real solution is added to circumvent the hashing problem.
Issues: #21, #121, #282

[bzz]

--help might need to be updated as well.

One more thing - on a supervised trained mode, this command prints first the word vector and only then the n-grams. Out of vocabulary word vector in this case becomes all 0, which if I'm not mistaken, seems to be inconsistent with print-word-vectors behavior.

[englhardt]

@bzz Thanks. I updated the help commands.

For your second point, I only adopted the current behavior of print-ngrams, i.e. method ngramVectors(...). I suggest fixing this in another PR.

[bzz]

@englhardt makes sense. Seems like rebase is also needed now.