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.
@ebolyen, HMMs are stochastic FA but I'm not well versed enough on the theory to comment on the specific relationships between HMMs and CFGs. However, Sean Eddy's group are experts with SCFGs and HMMs, and they opted to use HMMs for HMMER and PFAM. My guess is for performance reasons.

Unless I'm mistaken, the implementation detail does impact the use of `find_features` unless we do not pass `_sequence` to the consuming method (which makes sense for encapsulation).