Support `choice` provider

I want to be able to select any value from a given sequence.

Like:

mimesis('multiple_choice', items=['a', 'b', 'c'], number=2)
# => ['b', 'a']

mimesis('multiple_choice', items=['a', 'b', 'c'])
# => ['b']

mimesis('choice', items=['a', 'b', 'c'])
# => 'a'

It seems to be imposible right now.

[lk-geimfari]

@sadielbartholomew Deal! We’ll happy to merge your PR. Thank you and happy hacking!

[sadielbartholomew]

I’ve started to implement this (it’s currently not correct/finished so please do not review it yet) in a branch here. I’ll complete it, including editing the tests & documentation accordingly, in the next few days & put up a PR.