Notify about outdated git deps

[vincentdephily]

When specifying a dependency using git and a version-like tag:

[dependencies]
regex = { git = "https://github.com/rust-lang/regex", tag = "1.3.8"}

I would like cargo-outdated to

• Recognize that the tag is a version number (with some flexibility, like a v prefix or a -beta suffix)
• Look for similar tags in the git repo, and notify me about new versions

AFAIU, this is essentially how dep updates work in Go, and it would sometimes be useful to have that workflow in Rust.