Allow abstract snapshots in configuration, as with `--snapshot`

I want to be able to put in stack.yaml, something like "resolver: lts-5" and get lts-5.5 right now, but once lts-5.6 is released, have that be used.

My reasoning is, LTS is supposed to avoid breaking changes across minor releases, so it should be safe to switch to a new minor release most of the time. (Of course, mistakes and PVP violations can happen.)

I want to get security fixes in libraries I use (such as the tls library) pushed out to users promptly. I could just update my resolver lines each time a new LTS minor release comes out, but that's busywork that I'd need to keep up with on a nearly weekly basis.

So, having a way to get this automated would be quite useful.