Only show the toolbar button if the enableToolbarButton pref is set.

This should fix issue #53, by using a pref instead of an env variable, which was a previous suggestion. Can anyone else verify that it works, because I'm juggling too many balls atm and I didn't test it extensively?

I have flipped the pref locally in a prefs.json file that I load using the jpm parameter --prefs <path-to-prefs.json>
Nah, that makes sense - can we add quick instructions for toggling this pref to README.md?