Use a local random generator

[naslundx]

After #35 we touch the (global) random state with the seed argument. If the user wanted to do that, they could just call random.seed themselves before calling make_word_cloud. If we instantiate our own random generator and only seed that one, then this argument only affects randomness within wrdcld, and nothing else in a user's library or script. They may want "true" randomness elsewhere but a handy way to get determinism from wrdcld.