Show valid values for [SNAPSHOT]

[3noch]

[SNAPSHOT] shows up many places, but it's quite hard to figure out what are valid values for this. Somehow they need to be reported. At the very least, a link to an article or something.

It would also be nice, when a default is provided, for commands to report which default was picked. For example:

$ stackage init nightly
Initialized cabal.config for Stackage snapshot "nightly/2015-05-14"

[chrisdone]

Yep, agreed.