cli: Add app delete to CLI

Signed-off-by: Wang Baoling <chinamerp@gmail.com>
No, there's usually one app per folder, but it is indeed useful to be able to delete any app, however we can just do that with the builtin `-a` flag (`flynn -a controller ps`, etc.) Another thing to watch out for is that you remove the git remote, something that should not happen if a different app is deleted.