Update toggle when permissions change via events

[fregante]

While actions on click are being discussed in #33, there's an additional sanity check this module can run:

chrome.permissions.onAdded.addListener((newPermissions) => {
	1. get current active tabs (one per window)
	2. check if they match the new permission
	3. update their icon
});
// and onRemoved

This handles:

• new permissions via webext-domain-permission-toggle
• new permissions via other .request() paths
• removed permissions via Chrome's UI
• HOPEFULLY also permissions removed via "access site on click"

[aspiers]

Just for reference actions on click are also discussed in fregante/webext-permissions#22.