Add tags to jobs and a command line arg to select them

[Jamstah]

This allows the user to selectively run jobs based on a tag for use cases where you might want to run urlwatch on different schedules for different jobs and other use cases.

This is also part 1 of addressing #507 - next up would be allowing the report config to be an array (retaining existing behaviour for maps) and allowing users to assign tags to each reporter.

I am aware this will conflict with #785, and will rebase as required ;)

[Jamstah]

Thanks, appreciate the thorough review, I still have python reflexes from before 3.x - don't use it often!