Add ability to specify random number generator for determenism

[Erotemic]

In my application I need the new colors chosen by distinctipy to be deterministic. To accomplish this I simply added a keyword argument rng (for random number generator) to the appropriate functions.

In the default case when rng=None, the global random._inst random state is used. This means that this change is backwards compatible.

More interestingly, the user can either pass an instance of random.Random if they have their own seeded random state, or if they pass a constant value a new random state is created and seeded with that value.

This means I can do:

from distinctipy import distinctipy
distinctipy.get_colors(100, rng=1670706)

and that will return the same colors each time.

[jack89roberts]

Thanks again 🙂