Save artboard(s) to palette
Hi I was thinking of a huge improvement that could be made to your Sketch plugin that would be incredibly useful for anyone that is using overrides within Sketch symbols.

Essentially the problem occurs at this point
![image](https://user-images.githubusercontent.com/29231011/43083752-2327819a-8e8f-11e8-87b5-667f79749f11.png)


Where a user may have 40+ colors to add to a palette

Now my current method of handling this, and as far as I can tell this seems about the fastest way to do it is to add rectangle then manually add each to your document via the color picker.

![image](https://user-images.githubusercontent.com/29231011/43083835-52514a3c-8e8f-11e8-9328-aebe34bccdd4.png)

This is very time consuming and tedious. What I thought might be an awesome upgrade for people using this method of override-able colors would be to "Select color from artboard and add to palette". So essentially selecting the art-board and identifying whatever the color is within and adding to the palette.

I don't have any background as to what is achievable within a Sketch plugin, but if there's anything I can provide to help if you were to develop this, I'd be happy to help. 