Add scrolling/continuous-scrolling functionality

[saihaj]

Ideas:

• Tap lines to set speed (basically program notices line length and speed of "activating" line, and calculates speed based on that)
• Use line length to help refine speed, don't scroll too fast if lines are filling up screen, but speed up scrolling if lines don't wrap and have paragraph breaks, etc.
• Make sure to not scroll if starting from top of content (assume first most line is active and has to be read through)
• Indicate speed somehow (perhaps indicate which line is being read with a highlighted background and fill it up slowly as it's being "read" by program, this would be annoying if speed is off and keeps highlighting the wrong line though)
• need to know the height of words based on font size and the viewer port height (as font size increases, page scrolling speeds up too), track font size in implementation

[bhajneet]

Related to spec outlined in shabados/presenter#71 (comment)

[bhajneet]

Can be done without reader mode. Scrolling to just read more easily, and locking to set read speed could be an option too per hyper-button in drawer.