Allow to configure the text color from the CLI

[ggrossetie]

Currently, it's possible to configure the background color and the fill color but, as far as I know, it's not possible to set the text color from the CLI.

For instance, if I want to invert the colors:

White background with black text

$ echo 'test' | svgbob

Black background with white text

$ echo 'test' | svgbob --background black --text-color white