3nprob

A first attempt in #602 . Feedback welcome.

@callumgare if you're up for it, testing of #602 is welcome :)

FYI force-push was just rebasing with master and resolving merge conflict in docs

Sure thing, keeping in mind that `NotReplicated` has filesystems but no regex (unless you're OK with adding Regex to it?) so will still have the explicit field

> Hm, actually, can we just check in the `KeepNotReplicated` constructor function that the regex is nil? Do you mean that you'd like to see all the prunerule constructors take...

Ended up going along with that, also breaking out the `MustKeep*` functions to clarify that they're used as test helpers and not part of the application flow, and addressing an...

Just pushed an optional 195637b987d9c51439ab2af1ca0f6897db85c79e. I don't see why the other rules that support regex should not also be able to support negation. Just figured I might as well accommodate...

While there is discussion regarding CalDAV in comments in #4, the issue itself is only regarding ICS file support. Maybe the issue can be changed to indicate that it also...

``` Signed-off-by: 3nprob ```

OT but since there is no issue tracker on this repo to file a meta issue: the `CONTRIBUTING.md` link in the PR template is 404ing.