Allow buttons to take tasks as commands

[nicolaspayette]

This was made possible by @qiemem's work on #149.

It would be worth considering replacing the commands property by something like an on-click event, for coherence.

As mentioned on #149 (comment) :

One caveat is that xw:export/xw:import won't work for tasks (NetLogo/NetLogo#22), but I think we can live with that if its documented.