Add syntax file for '.PKGINFO' and '.AURINFO'

1. Add file syntax/PKGINFO.vim for '.PKGINFO' and '.AURINFO'.
2. Parse .INSTALL as shell filetype.
Refine plugin/PKGBUILD.vim

1. Refine plugin/PKGBUILD.vim
   Add 1 command -- :MkAurBallForce
   Refine 2 commands -- :UPS and :MAB
2. Update README.md