Add command line option for setting time regex 

I'm working on a Webthing implementation that uses more accurate timestamps including milliseconds. 
E.g. ```2023-07-16T21:41:16.123+02:00```

Therefor it would be nice to make the testers ```_TIME_REGEX``` variable configurable via command line like the other variables (```_PATH_PREFIX```, ```_PROTO```, ...)