Tail process buffer if the point is at the end

Closes #53. Does this seem like an OK approach?
This is because of slightly weird behavior with `with-current-buffer`: when the buffer is visible in a window, the point won't update for the window (that's why I originally couldn't get tests to pass). This way, if there is a window with the buffer open it will switch to it so the point gets updated (let me know if you know of a better way).