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.
My thinking was that this method (```find_features```) should intended to make searching easily-describable features (e.g., the purine runs or pyrimidine runs currently implemented) convenient.  To facilitate searching for repeat regions of arbitrary nucleotide patterns, there is the more general ```regex_iter``` function in the base class.

With respect to finding strings of aromatic AAs, I could add a ```find_features``` function to the ```ProteinSequence``` class, and add an "aromatic AA run" feature.