Add support for --locked flag

[tgnottingham]

This is a feature request to add support for a --locked flag which would cause cargo-machete to produce an error rather than inadvertently modify Cargo.toml files (currently only applicable when --with-metadata is used).

[bnjbvr]

Thanks for opening an issue, that makes sense to me. In fact, I seem to recall I've tried to make the behavior you're suggesting here the default (as there's really no reason that machete will update the lock files), but I didn't manage to do it, for some reason I don't remember.