e621 Subscriptions

Adds Watched/Subscribe feature to e621.net

Extension adds subscription to tags functionality for e621.net, adding "Watched" button into navigation bar at the top of the page and embedding "OwO/XwX (Subscribe/Unsubscribe)" button for every tag shown on the sidebar. There is also an option to check new images without visiting the site -- just open extension's popup window and it will show amount of new images.