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.
This might make more sense as `import re` then it becomes `re.compile`. Though I don't know if we have standards for this to indicate the use of one over the other.