Expose label background color and hover color in settings

[Bytenex]

Hey,

currently it's not possible to color the label color when hovered or the background which makes it very hard to use on dark background. Currently I have set the font color to grey so it's visible on the dark background and the white hover background. As labelColor is already an object it would be suitable to add these settings to.

labelColor: { color: '#fff', hover: '#000', background: '#fff' }

[Yomguithereal]

@Bytenex this can be dealt with using a custom label renderer such as this one here: https://github.com/medialab/ipysigma/blob/master/src/custom-label.ts (or if you want to customize the hover part: https://github.com/medialab/ipysigma/blob/master/src/custom-hover.ts)You can easily make your own to add support for a background color. Then you can pass those functions to your sigma instances in the settings as labelRenderer & hoverRenderer.