Add support to list outdated dependencies in the Godeps file

[thomasdziedzic]

Something along the lines of bundle outdated that ruby has but for go. This would help a lot in terms of keeping up to date with the latest versions when using version constraints in the Godeps file.

[pote]

I could see this functionality being implemented as a plugin, it shouldn't be too hard to do, as it's basically running gpm (to get the latest version of all packages) and walking through src/ in the first directory in the GOPATH to compare the current identifier to the latest one.

I'm not sure I'll have the time to implement it myself, but if anyone here wants give writing a gpm plugin a go, then now is the time! 😄