Allow parsing timestring at different timezones.

[knobo]

Allow parse-timestring to parse timestring in different timezones. This would be useful in cases where timezone is sent separately like it could (and often does) in the icalendar format.
As discussed in #56

[orivej]

I do not maintain this repository, but I like this feature, and I think that you should update the docstring, add a test to test/timezone.lisp, and probably avoid apply by relying on encode-timestamp to ignore :timezone when :offset is not nil.

[knobo]

Use #58 instead.