* custom.el (defcustom, custom-handle-keyword): Add :package-version

* custom.el (defcustom, custom-handle-keyword): Add :package-version
keyword.
(custom-add-package-version): New function. Sets value of new property
'custom-package-version from :package-version keyword.

* cus-edit.el (customize-package-emacs-version-alist): New variable.
(customize-changed-options): Add check for custom-package-version.
(customize-package-emacs-version): New function to look up Emacs
version corresponding to the given package version.