Add a way to update a thing installed by `cargo install`

[staktrace]

See https://bugzilla.mozilla.org/show_bug.cgi?id=1336555 for backstory. It would be nice to have a way to update a tool installed by cargo install to the latest available version of that tool. e.g. running cargo install --update cargo-vendor would download and install the latest version of cargo-vendor, but only if your version is out-of-date.

[est31]

Dupe of #3496 and #2082

[alexcrichton]

Indeed, I'm going to close in favor of #2082