cargo dependency subcommands

[wesleywiser]

Hi,

I had a few ideas for new cargo commands, I think they're fairly self explanatory:

cargo dependency add nickel --git https://github.com/nickel-org/nickel.rs.git

cargo dependency rm nickel

cargo dependency lint
Checks for unused dependencies, out-of-date dependencies, etc.

Thoughts?

[alexcrichton]

An example of adding a dependency can also be found at #4. The hard part about writing to a manifest is preserving areas such as comments. Currently toml-rs isn't fantastic about maintaining comments.

That being said, it'd be pretty awesome to have this!

[alexcrichton]

I think https://github.com/killercup/cargo-edit serves this quite well!