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>
It's a hidden pref in the manifest, so it doesn't appear in the add-on manager. I didn't think it would be worthwhile for non-FD devs to toggle it, so I left it out. I can add it back if you think it would be useful.