cli: Add app delete to CLI

Signed-off-by: Wang Baoling <chinamerp@gmail.com>
@chinamerp so if you use `mustApp`, the api would be as follows:

```
# delete the current app (as determined by the flynn git remote)
$ flynn delete

# delete the app with name "example"
$ flynn -a example delete

# delete the app with ID "flynn-abc123"
$ flynn -a flynn-abc123 delete
```

If you are worried about the user deleting the wrong app by mistake, you should add a prompt like:

```
Are you sure you want to delete the app "example"? [y/n]
```