Feature request: Way to change scroll

[mcclure]

Inspired by #91. Currently you can move the cursor, and you can get the current scroll position, but you cannot change the scroll position. A move_scroll() with Jump(u16) and Forward(u16) would be great.

(Note: I see that you can currently influence scroll with move_cursor(), because moving the cursor offscreen will scroll the cursor. However this isn't quite good enough because you might have opinions about where on screen the cursor should be, like scrolling to put the cursor in the middle of the screen would be awkward. I also do not know what I expect to happen if you move_scroll() and it moves the cursor offscreen, as this is currently impossible.)