cask/postflight: Run generic QuickLook-related code only when necessary
cask/postflight: Run generic QuickLook-related code only when necessary

"Necessary" means if either -a,--all is specified or at least one the
newly installed casks has "ql" or "quicklook" in its name.

This commit paves way for the restructuring of cask/init: postflight
will be called after the completion of installation of each cask.

zmwangx/dotfiles,zmwangx/dotfiles,zmwangx/dotfiles,zmwangx/dotfiles,zmwangx/dotfiles