Add keyboard-support with three modes:

1. none: No left/right key support.
2. focus (default): It only focuses the option when user navigates to it using left/right keys.
3. select: It selects the option when user navigates to it using left/right keys.

Other requirements:
Needs to cycle through options when reaching the end.

Also See ampproject/amp-by-example#697 (comment)