Implement partial updates

[djmitche]

In many cases, an update only changes a few lines -- an integer increments here, a status changes over there -- yet log-update erases and redraws the entire output. For large ouptuts over a remote connection, this can cause some flickering as the screen refreshes between the erase-everything sequence and the replacement output.

In these cases, log-update can easily determine which lines have changed and only update those lines. #29 is a start at this, optimized for the case where the updates are near the bottom, but we could do better.

The approach is sort of like the invalidation-rectangle approach used by browsers to decide which DOM elements to redraw -- only much simpler in this case!