Support for layer styles
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**Feature description**
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Unfortunately, document colors that html-sketchapp generates are not shared when using a sketch file as a library. (The only things that libraries provide are symbols, text styles, and layer styles)

It would be cool to have a generator for layer styles similar to how text styles are implemented. 

E.g. our style guide specifies that a card has a light grey background, a dark grey border, and rounded corners. In Sketch I can save these for reuse, and I would like to generate them automatically. 

![screenshot 2018-11-15 17 37 48](https://user-images.githubusercontent.com/1100018/48586177-38ac8380-e8fd-11e8-8862-c81170be7dad.png)

