Enhancement - Add smart remove like BIT

[dkebler]

enhancement

the maxsnaphosts is an ok management for now but it would be nice to have something more like what BackinTime has

for starters just a --smartremove which would keep
all in current day, one a day for last 7 days, one a week for last 4 weeks, one a month for last 12 months and one a year. (by default)

then later allow those defaults to be customized

[dkebler]

@mattlyons0 not throwing this necessarily on you. I could come up a PR (might be later this winter). Just want to list enhancements I would like to see. This is a great project (needs more stars) as an alternative to BIT which is really only a GUI snapshot solution. A few enhancements might make more popular.

[mattlyons0]

I've considered adding a flag to delete all snapshots after a certain date, this would be an extreme version of that. I'm open to a PR if you can implement it in a clean way.