`jump-start add` to create a new starter

[kevinschaul]

I want to make it easier to add starters. I'm thinking some subcommand like this could help. Maybe it should also be tied to the editor plugins.

jump-start add <starter_identifier> FILES [FILES]

Some design questions:

• How to handle creating jump-start.yaml? Could we generate a generic one? Or open it in a default editor?
• Should it auto-commit? Otherwise it will be easy to have dirty starters. (Related, should there be some jump-start update command that runs a git pull on all instances?)