Advice: Consider adding some features from stefanhusmann/pkgbuild-mode

[Firef0x]

考虑从 Emacs 的插件 stefanhusmann/pkgbuild-mode 抄袭一些特性。
见 https://github.com/stefanhusmann/pkgbuild-mode/blob/master/pkgbuild-mode.el