Add ignore_unspecified option to PropertySortOrder

Add option `ignore_unspecified` to `PropertySortOrder` which allows
properties that are not explicitly specified in custom sort orderings to
be ignored.

This makes the linter more flexible and easier to use since users don't
have to specify every single CSS property in existence in order to make
use of custom orderings.

However, to preserve existing behavior, the default value is set to
`false`.

Change-Id: Ie9b5094a26d161a56ae8c7cf8f28654dcbda831a
Reviewed-on: http://gerrit.causes.com/40840
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>