Finish implementing/documenting !link syntax

[tve]

(Would be nice to document the bunchfile syntax fully)

[dkulchenko]

!link is meant to be a more general-purpose !self - it allows setting up a vendored symlink to a local repo rather than checking something our from Git/other VCS as go get does.

It's undocumented because it's untested - I'd like to finish it at some point, though.