Add filetype detect file and 2 commands

1. Add file ftdetect/PKGBUILD.vim to detect filetype.
2. Try to parse .AURINFO as PKGBUILD filetype
3. Add 2 commands -- :UpdPkgSums and :MkAur
   Refer to https://github.com/Firef0x/PKGBUILD.vim/issues/2
   and https://github.com/Firef0x/PKGBUILD.vim/issues/6
4. Update README.md
Refine templates/PKGBUILD
Add 2 new keywords of 'options' for syntax/PKGBUILD.vim
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
   Refer to https://github.com/Firef0x/PKGBUILD.vim/issues/2
   and https://github.com/Firef0x/PKGBUILD.vim/issues/6
2. Update README.md