Add offline mode (fallback to env)

I tried to add an offline mode using `-o` (as suggested in https://github.com/tpope/vim-classpath/issues/6 ), however lein respected it only for the stuff from the maven repos while still downloading from clojars.
Also maven's dependency plugin outputted a lot of error messages with `-o`.
So I opted for defaulting to the fallback (`$CLASSPATH`).

Ideally this should be done in a more flexible way (e.g. falling back on cache, adding a command for updating the classpath, detecting when you are offline, etc.).