default git remote, stored in git config

store it with `git config gordon.origin "docker"` or whatever your
upstream is named. If this value is not configured, then "origin" is the
default.

The `--remote` flag will still override this configuration.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@hashbangbash.com> (github: vbatts)
Merge pull request #147 from vbatts/vbatts-configurable_remote

default git remote, stored in git config