Phrase support baed on word2phrase

First approach to learning phrases based on wordphrases. Please comment. Not test and proper documentation yet. But they will come. 
Didn't check the original, but shouldn't `last_bigram` get reset too if this condition fails?

Like this, `last_bigram` never gets reset to `False`, until line 138 happens, which can be after many words, far away from the bigram?