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.
Hmm I'm not sure I understand the request.  Without those things hard-coded, how would the caller specify what features should be retrieved?  I suppose each could be its own instance method (e.g., have a method for ```def iter_purine_runs(self, allow_gaps=False)``` and likewise for ```iter_pyrimidine_runs```).  If I'm still not understanding, I'm sorry.  Can you (or others @gregcaporaso @jairideout @ElDeveloper) suggest an alternative implementation?