Allow --not-when-* flags to apply to one timer

[sidequestboy]

I had suggested in #41 that we extend the syntax of --not-when-* flags to allow specifying for individual timers. The original suggestion was:

xidlehook [FLAGS] [OPTIONS] --timer <duration> <command> <canceller> [--not-when-audio][--not-when-fullscreen]

Where OPTIONS includes

--timer <duration> <command> <canceller> [--not-when-audio][--not-when-fullscreen]

and FLAGS includes

--not-when-audio
--not-when-fullscreen

so that if a --not-when-* flag follows a --timer option, it applies only to that timer, and if it precedes all --timer options, it applies to all timers.

I'll try to make a PR about this once I figure out this rust stuff.