use tok as fallback tokenizer

[kootenpv]

The tokenizer makes for a better default (faster and saner).

Targets #10

[soaxelbrooke]

For context, in NLP tasks it can be important to retain the usage of contractions, since it can be informative about other aspects of the author of the text.

[kootenpv]

@soaxelbrooke I personally think the potential benefit of retaining contraction information is more than compensated by the increased power of generalization by having it all normalized :)!

i.e. not and ' / t are not generalized, did and didn don't generalize. I deem this to be worse!

I'm up for further discussion. Meanwhile, how you would propose adding it as an option without also having the nltk dependency?

[soaxelbrooke]

@kootenpv I'm fine with people having different preferences on how they'd like those split up, my biggest concern here is changing the interface, which is a breaking change that would introduce bugs into dependent code. I haven't heard any complaints about the presence of NLTK, so I don't see any particular need to remove it. We could expose tok as an alternative word tokenizer in the Encoder constructor.