Consider using external generator by default

[NikitaVlaznev]

Many people need slug generator, small part of them need a mongoose slug generator, it looks to be clever to use some external library as a default generator. Consider using https://github.com/dodo/node-slug for example, or something else. With exernal library generator issues will be a matter of a larger community, while you can focus on the integration with mongoose. You will have less issues like #31.