Add -recursive option to the update command

[rohanthewiz]

Bundler has bit me several times when trying to update a single gem. It defaults to updating dependencies that don't necessarily need updating. Refer to this article: http://ilikestuffblog.com/2012/07/01/you-should-update-one-gem-at-a-time-with-bundler-heres-how/.

What is gvt's dependency management philosophy? I see you have the ability to specify refs.

[FiloSottile]

Thanks for making me think about this.

gvt has very little opinions on how dependencies work, which means it has to be pretty dumb about it. The current behavior is what you advocate for, and it will stay that way. gvt update name will only update name.

I'll add an optional -recursive mode now to update a package and all its dependencies, which is the bundler behavior.

We can't do anything smarter (i.e. only update when needed) because there's no concept of acceptable versions in Go.

[FiloSottile]

(Also, update should only warn on fixed dependencies when doing -all)