Add choice of monitor system app

[rcspam]

Hi, It could be nice to choose an other system-monitor than the one by default (i.e. htop in place of gnome-system-monitor with unity) with a command line option in preferences.

Another idea is to open a dedicated app when click on a graph ( ex: nvtop on GPU graph, apachetop on net graph....), except if gnome-shell-extensions kit doesn't allow this kind of stuff...

Thanx for your work...