Support rules for transitions beyond the last explicitly listed one

[ygale]

Add a field to TimeZoneSeries containing a rule to generate transitions arbitrarily into the future beyond the last explicitly specified transition. This would enable:

1. Support such rules that now exist in binary Olsen timezone files. This would only require supporting POSIX rules.
2. Support parsing a TimeZoneSeries from a tzdata source rile. This would require supporting more general rules.

See comments in ygale/timezone-olson#3, submitted by @AshleyYakeley.