Allow specify index-state per repository

[phadej]

One concern I have is the syntax.

v2-update uses comma :

hackage.haskell.org,HEAD
hackage.haskell.org,2020-03-16T01:24:50Z
hackage.haskell.org,@123123123

Yet, I don't want comma in configuration files to mean anything else then list separator,
therefore I took colon, which we use e.g. in allow-newer and other "namespace" cases:

hackage.haskell.org:HEAD
hackage.haskell.org:2020-03-16T01:24:50Z
hackage.haskell.org:@123123123

This is a bit unfortunate, as timestamp has colons in it.

Another alternative would be to use @, note double @ when using posix-timestamp.

hackage.haskell.org@HEAD
hackage.haskell.org@2020-03-16T01:24:50Z
hackage.haskell.org@@123123123

---

I'll merge the colon version; this can be changed before 3.4 is released.
Perfectly, v2-update would use (or at least accept) the colons too.