Make nodeToSketchLayers recursive
`nodeToSketchLayers` should run recursively and generate layers for all descendent nodes. Two reasons:

- that's more intuitive (e.g. #66)
- order in which nodes are processed is important, if we leave it to the user she/he may get subpar results (proper node traversal can be found here - https://github.com/brainly/html-sketchapp/blob/master/test/inject.js#L16 ).