New port: devel/govendor

Govendor is a tool that fetches dependencies for Go projects and records them
in the standard vendor.json file.

WWW: https://github.com/kardianos/govendor

PR: 212514
Submitted by: Randy Westlund <rwestlun@gmail.com>
Reviewed by: mat
Differential Revision: https://reviews.freebsd.org/D7818