Refine plugin/PKGBUILD.vim and update README.md

1. Refine plugin/PKGBUILD.vim
   Add 1 command -- :MkAurBallForce
   Refine 2 commands -- :UPS and :MAB
   Refer to https://github.com/Firef0x/PKGBUILD.vim/issues/2
   and https://github.com/Firef0x/PKGBUILD.vim/issues/6
2. Update README.md, add links to Arch Linux packages