Add: initial-index keyword argument to function `popup-menu*'. (initial-index argument is optional argument)
Usage:
  (popup-menu*
   '(0 1 2 3)
   :initial-index 2)
Merge branch 'initial-index-keyword-argument' of github.com:kiyoka/popup-el into initial-index-keyword-argument
Revert "Merge branch 'initial-index-keyword-argument' of github.com:kiyoka/popup-el into initial-index-keyword-argument"

This reverts commit 8ed4cbcddf59b553093f6b89d92a0e0351b1dec4, reversing
changes made to 3009969594c52c69afb0b0a8cb9d272dd71507a6.