Set available patterns

You provide a large list of beautiful patterns. For my usecase I want to use just a subset of all available patterns. This PR makes it possible to reduce the available patterns as optional argument to ".generator".

BTW:
Is there a test suite available anywhere?

Cheers,
dg
I don't think this section is really necessary since the library itself isn't picking random colors. 

However, your proposed feature is for passing in an array of generators which you didn't include here.