Prototype for 'purge' command (#16)

[excavador]

I prototyped 'purge' command, for that I copied & fixed related file from gb-vendor. I am not sure how exactly 'ParseImports' should be used for this tasks. Hope for your help to complete the task

[slomek]

@excavador I guess you need to add pkg/errors to vendor, then the Travis checks may pass.

[excavador]

@FiloSottile could you please provide any feedback how exactly it should be fixed?

[slomek]

@excavador gvt fetch github.com/pkg/errors on your branch and add all updated files.

[excavador]

@slomek I do not sure what that's the right solution (if you compare with other files you''ll pkg/errors used nowhere). That's why I am asking the owner of the repo, I need him opinion about the "right" way how he see it.