Add ability to create global colors on documents

[hadihallak]

this functionality seems to be missing in the document model

[L2jLiga]

Hi there, I think PR will be pretty appreciated :)

[chocosd]

Would be nice to have the documentation to update Color Variables as well; as all the documentation seems to show us making new things on existing documents, but not replacing things. I can sketch.getPage and .getArtboard and getGroup and even access the Swatch object, but i can't see where you would redefine anything that's existing