Add `--paper-checkbox-checkmark` CSS variable

[admwx7]

I've got some styling requirements to show partial selection where instead of a checkmark the item has a small box inside, it's still triggering all of the standard situations of a checked item so the checkmark is showing up, and I can change around colors when in the "partial" state, but I can't change the checkmark to be a box instead of a checkmark without using global CSS styles, a simple mixin in the #checkmark CSS would resolve this issue.