start work on pushing fill attributes down to subtrees with only loops

See diagrams/diagrams-svg#43.
implement splitFills for case of style node
finish implementing splitFills: RPrim case
D.Attributes: fix warnings
D.Attributes: clarify comments on splitFills
finish implementing splitFills (with tests).
Merge pull request #161 from diagrams/split-fills

Move FillColor Attributes to subtrees with only Loops