Add command to quickly create a pull-request

This command is mostly made for fork->source merge workflow. When you run the command, a quick panel will appear asking to which origin you want to create the pull request.

Also this command implies that branch names are the same in both the fork and the source. This feature has been tested in both Sublime 2 and 3.

Ideas for future improvements:
- if there is only 1 remote, the quick panel should prompt to select a branch, not the origin

What do you think?