Improve the semantics of --after/--before

[jotaen]

This issue is a spin-off of https://github.com/jotaen/klog/issues/27. --after and --before are currently inclusive, which is not intuitive. On the other hand, there could also be legitimiate use cases for it, e.g. --after=2000-01-01, instead of --after=1999-12-31. I think the ultimate solution is this:

• Make --after and --before exclusive
• Offer a separate inclusive version of the flag e.g. --after-or-at or --after-incl --since and --until
• Support date ranges for convenience, i.e. --in=2018 for “all in 2018” or --in=2018-01 for “all in January 2018”.

All exact names tbd., suggestions welcome.

[vladdeSV]

Just a suggestion:

• --since
• --until

[wAuner]

@vladdeSV that's what I wanted to suggest as well. make after/before exclusive and since/until inclusive.