ENH: Add iter_regex and find_features

Fixes #612. Adds iter_regex to skbio._sequence.BiologicalSequence and
find_features to skbio._sequence.NucleotideSequence. Supported features are
currently purine_run and pyrimidine run, but more can be added by specifying
additional regular expressions.
You are correct, but much faster to search for motifs like AATTRRYG but will not support (IIRC) repeats. 

I like the ability to pass a function, and have optimized hardcoded sets

I agree that you can rapidly convert a numpy array to a string as numpy (though I observed 64us for a 1mil string using `np.array.tostring`) and that it does not block using regex as I had indicated. But, the underlying type is necessary to resolve if we support arbitrary functions otherwise we don't know what the signature will be. Or, we ignore that and eat dev effort down the line if we change the representation but that has potential to impact how the API is being used, so that could be very annoying for packages that depend on skbio.