Fixes issue #98

1. Changed dragger/ gutter from div to button in template for keyboard focus
2. Added "startKeyboardDrag", which uses new util functions to calculate the start and end points differently from mouse events.
3. Moved chunks of "startMouseDrag" out into separate functions so that keyboard drag can share the code: this.currentlyDragging(), this.drag();, this.stopDragging();

Question: Thoughts on getKeyboardEndpoint doing the "exit early" checks and returning null? Should that be handled by a separate function?

Issues discovered while developing:

1. tab order of the gutters is not top-down left-right
2. keyboard resizing does not work while in screenreader mode (stopPropagation could be blocking it)
3. should add a new binding to set an aria label on the gutter. Could be addressed in a quick followup PR targeting screenreader support

Note: The vertical splits in the demo gif don't have the smooth animation because they don't have a the useTransition animation binding set to true