Add "archive" as an alias for "archive-z2"

Add "archive" as an alias for "archive-z2"

I find the "-z2" is really a long ago relic of the past when I changed
the format.  We no longer have anything to do with the original
`archive`, so let's start allowing people to type `--mode=archive`
which just looks saner.

At some point later I'll update the docs too, but it'll be an annoying
transition period as we'll have to say "On older OSTree, use -z2" etc.

Closes: #346
Approved by: giuseppe