image formatter supports background color (#1374)

Extend Image formatters in img.py to support background colors.

* New function to get the background color for a given token type, optionally None
* The self.drawables list used the fill keyword, stuffed into a **kwrgs, for the color of the token; since tokens can have foreground and background colors, that list now uses two explicit names, passing one to the PIL/Pillow text draw call, the other to the rectangle draw call.
* When iterating over the `drawables` list, the presence of a background color triggers the drawing of a rectangle beneath the text.
* Dimensions of background rectangle calculated by PIL based on the rendered text and font.
* If input string is multiline, PIL will use multiline for draw and size internally.

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