Feature request: only display non-empty spaces

[cesarolea]

I keep 10 spaces at all times to avoid having to create and delete spaces on demand. It would be nice to have an option in Spaceman so it only shows non-empty spaces, similar to how i3 works in Linux. So for instance if I have 10 spaces but only space 1, 2 and 10 have windows in them it would show [1][2][10]