Improve lint message for Shorthand linter

The shorthand linter previously gave no instructions for how to fix the
lint other than saying that the shorthand property value needed to be in
the shortest form possible.

Change the implementation to calculate the shortest form while checking
the values so that if they don't match up it can show the best possible
form.

This saves the developer from having to think when they see the error
message, which is a huge benefit.

Change-Id: I277f71c15f375d8db92bfdc8273ab94e1aba725b
Reviewed-on: https://gerrit.causes.com/29071
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Noah Silas <noah@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>