Add optional background to a map label

[rchowell]

See updated doc/reference.html.
See added examples/maplabel-background.html

Added:

• backgroundColor
• backgroundStrokeColor
• backgroundStrokeWeight
• padding
• borderRadius

Not specifying a backgroundColor will cause the map label to operate exactly as before. I have given the above additions sensible defaults, but feel free to critic and change. Also, I did not add redraw on update of any of the follow added properties. If you feel like that is needed to complete this pull request, then I will add and test.

Best,
rchowell

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