Send alerts to the terminal on events

[mathstuf]

Probably should be a configuration setting, but making a bell on the terminal when an event happens would be great (I'm using GitLab terminology here, I'm not too worried about naming):

• pipeline-started
• pipeline-failed
• pipeline-completed
• job-started
• job-failed
• job-completed

[nbedos]

I agree that this would be useful but it is currently not possible without fixing gdamore/tcell#2 first. This PR should be enough for Unix systems so I'll probably have to make a private fork of tcell that includes this change.

I'll wait a bit before working on this though, in case tcell has other limitations that would make using ncurses a better choice.