`tokenize` API

The `set_tokenizer` API seems a bit suspect here, given that it can be replaced with
```julia
const tokenize = WordTokenizers.nltk_tokenize
```
and likewise for RevTok etc, without bringing in multiple packages just to define an alias :)

I also think it's generally a good idea to expose people to higher order functions and such; people might not realise that you can just e.g. pass a custom tokenize function into a constructor rather than setting and unsetting it globally.