Add option for font/stroke color

[len0rd]

Thanks for this great project! Continue to use this across multiple Sphinx doc projects.

This PR adds an option for specifying the color to use for all outlines/strokes/fonts.

For instance running something like this:

python -m bit_field --fontcolor "#FF4500" --lanes 2 readme_alpha.json

will result in this SVG:

you can also use svg color names:

python -m bit_field --fontcolor blue --lanes 2 readme_alpha.json

And the default remains as black:

python -m bit_field --lanes 2 readme_alpha.json

I realize adding more functionality to something that's meant to be a direct port is not necessarily desirable. So i understand if you dont want to merge this in. I saw a need for this to make my bitfield diagrams more legible on dark-mode sites.