Script to init the upstream remote for Marlin

[thinkyhead]

The other scripts require an upstream remote to be set for the main Marlin repo. This script ensures that it will exist. It may, in future, test for the existence of the remote before adding it, then the other scripts may call it. Note that it creates the remote in git mode and not http mode. If needed, use ghtp -h to change all remotes to http after creation.

• Oh yeah, so here's ghtp also.

Usage: ghtp -h | -s
 -h to switch to HTTPS
 -s to switch to SSH