Add slug words limit

[jesusbotella]

Hi!

I wanted to limit the number of words within the slug and I thought to add that to the script. I hope it'll be useful for someone :)

I made the test, and it works correctly ;)

Best regards.

[lillem4n]

Since this repo does not seem to be accepting pull requests, I've forked it, added this and some other pull requests and published another node module with the updates; https://github.com/larvit/larvitslugify I've added you to contributors because you contributed. :)