Expose the node element to symbolLayerMiddleware

[daynekilheffer]

We just started using this tool and while it seems to import nicely into sketch, we have encountered a few issues with regard to resizing. We have attempted to use symbolLayerMiddleware to resolve some of our issues, but basing constraints on what the layer object contains limits our ability to determine what/how constraints should be applied.

My suggestion is that the dom node used to create the layer be exposed to symbolLayerMiddleware to allow the user more control over configuration of the layer.
It will also align the layer middleware with the symbol and symbolInstance versions in that they already expose the dom node.

I can submit a PR of this change if it seems appropriate to the design of this project.