Support cargo workspaces

[euclio]

It would be nice to provide a flag to view outdated dependencies for the entire workspace.

[kbknapp]

Hmm I'm not super familar with cargo workspaces yet. Let me look into this once I've knocked out a few bugs and updated the code.

[mbrubeck]

rust-lang/cargo#3704 suggests that the best way to do this is by parsing the JSON output of cargo metadata rather than the Cargo.lock file. The main difference in a workspace is that there may not be a "root" package.

[SergioBenitez]

I just want to chime in and say that this would be incredibly useful! I use cargo outdated quite a bit. Rocket has plenty of example crates (~30), and it would be nice to use cargo outdated --all (or something similar) to check whether all of their dependencies are up to date.