skip custom functions in consecutive commands
cancel active processing on enter of command

* clean-up for the default 'find again' command when our custom functions are
  skipped
* see commit: 1d49172
remove useless comparing current and last time

* there is almost no difference in those with using animation frame
fix incomplete end handling of scroll
arrange code to specify workarounds
give a name to the anonymous function
minor fix

* use |let|
* edit comments
* formattings
improve animation function

* create a handler of frame animation: |FrameAnimator|
* use the animator to blink a selection instead of using |setInterval|