Balancing

Implemented the balancing construct (?<->) from .NET regular expressions.
Recursion Parser Installation Fix

Group and whole pattern recursion parsers no longer install themselves as capable of parsing within character sets.
Conditional

Implemented the (?()) construct from .NET regular expressions, which matches one of two subexpressions based on the result of some check -- either a check to see if a group has captures, or a check to see if a zero-width assertion holds at the current location.