Do not fail when remote origin already exists

[tradej]

When GitHub remote is added as origin, currently DA fails on whichever error. I believe it would be more beneficial if, in the particular case that remote origin already exists, DA asked the user if they want to name the remote differently.

I believe this makes sense in the use case where a user checks out a git repository not hosted on GitHub, but want to host their forked version on GitHub. In my particular case, it was urlgrabber, which is based on baseurl.org, and I wanted to push my changes to my personal GitHub repository, whose remote name I selected as github.

This is a clone of devassistant/devassistant#250