macOS: badge on dock icon

[ftpd]

In #7101 the new way of notification was introduced - on bell the dock icon will bounce one time. I would like to ask for adding a badge to the dock icon as well.

Use case: I'm running a ./really-long-command and I want to be notified when it finishes. So, I'm running ./really-long-commmand; tput bel and it should be fine - sure, but what happens if it finished while I was AFK?

The icon badge is an ultimate solution - when I'm back I know that my terminal emulator needs an attention. Bouncing icon is fine when assumed I'm always looking on the screen.

[mitchellh]

#7108. Thanks