Highlight current move in textarea while animating.

[lgarron]

I quite like how changing the textarea selection worked in 6804e40. It started down the path of using a cursor in the textarea to match with the animation (see #16 for the opposite direction). However:

• This triggers a keyboard on mobile, which can cause a layout change.
• It might occasionally be useful to keep the cursor in a single place while changing animating independently.

[lgarron]

Implemented for non-touch devices in 9bba559.