Feature: performance improvements

By adding [stemmer](https://github.com/wooorm/stemmer) and [metaphone](https://github.com/wooorm/metaphone), performance on the benchmark increases significantly.

The benchmarks were done on a MacBook Air (late 2011), while listening to music and such, but the difference is certainly there.

Results w/ `stemmer` and `metaphone`:

```
             139 op/s » tiny
              29 op/s » small
              10 op/s » medium
               0 op/s » large
```

Results w/ `natural.Metaphone.process` and `natural.PorterStemmer.stem`:

```
              96 op/s » tiny
              20 op/s » small
               5 op/s » medium
               0 op/s » large
```

P.S. As my result are, both with and without this feature, slower than the benchmark in `Readme.md`, I’ve not updated it.