`doing open` runs the editor and waits for it to exit. An editor that opens its own window never does, so the terminal stayed unusable until the user interrupted it. Add `process::spawn`, which starts the editor and returns, and reach it from `doing open --no-wait`. The default still waits, because a terminal editor needs the terminal to itself. Detach the child's streams when spawning. Inherited, they would keep our pipes open after we exit, so anything reading `doing`'s output would wait for the editor regardless. Closes #435