Hi Seth, thanks for the great work on this lib!
I'd like your opinion on something regarding the version sorting.

From https://github.com/SethMMorton/natsort/blob/master/test_natsort/test_natsort.py#L211

>>>  a = ['1.9.9a', '1.11', '1.9.9b', '1.11.4', '1.10.1']

Let's add the '1.11a' version to the list.

>>> a = ['1.9.9a', '1.11', '1.9.9b', '1.11.4', '1.10.1']
>>> natsorted(a)
['1.9.9a', '1.9.9b', '1.10.1', '1.11', '1.11.4', '1.11a']

I think the '1.11a' should be sorted before the '1.11.4'.
What's your take on this? Thanks.