Quiet verbosity effects

Created by: ascjones

Currently the --quiet flag is just passed through to the underlying cargo invocation (see https://github.com/paritytech/cargo-contract/pull/133#discussion_r552469809).

We should consider making --quiet not print anything to stdout as it does in cargo, whether by wrapping all our println!s in conditionals and/or investigate whether we should print progress updates to stderr as I believe cargo does (needs confirming).