Make snapshotExpand and snapshotEnv simpler/more flexible

[sewe]

Not sure whether this should go into the 1.x line or be slated for 2.0, but I think snapshotExpand and snapshotEnv are not very flexible while being a bit on the verbose side.

Why not have a single String(!) snapshotExpand which defaults to "SNAPSHOT", i.e., a no-op. You could then mimic the current snapshotEnv environment-variable behavior using

<snapshotExpand>${env.MY_VARIABLE}</snapshotExpand>

And if you want a time-stamp, you could use a property as well, generated, for example, by build-helper:timestamp-property. I'd rather use that as have n slightly different timestamps (in slightly different formats) used for the same artifact (if., for example, you let the maven-resources-plugin interpolate your ${myTimestamp} property into a resource file.

Thoughts?