Allow scrolling when the user scrolls to the edges in Region mode #255

Currently, we use tabs.captureVisibleTab() to grab a screenshot of the current viewport, and then compute the resulting Region screenshot with the user's start and end coordinates. See: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/tabs/captureVisibleTab

Since captureVisibleTab takes a screenshot of the viewport and NOT the entire page, in its current implementation, it would be tough to allow scrolling. To allow scrolling, we would have to investigate taking multiple screenshots during the scroll action, and 'stitching' the screenshots together before using the selection coordinates to extract the Region. This would be a pretty big change I think.