Minor issue + suggestion (grammar.py)

[Marcus Karr]

Hi all, I just joined the group and hope this is the appropriate venue for reporting a small issue.

I have found a problem in trying to call PCFG.fromstring() on string representations of productions taken from the Penn Treebank.  Some productions have the form . -> '.' or , -> ',' which causes a ValueError to be raised from standard_nonterm_parser().  Perhaps the pattern in _STANDARD_NONTERM_RE could be extended to cover single-character punctuation marks like the comma and period, since these are sometimes used as nonterminals.