feat: Add option to override default css rather than replace it

[bryanforbes]

Up until now, I've had to dump the config and add my custom css rules at the end of the default css rules in my config. When an update happens, I have to diff the default css rules to see what has changed. This PR adds a new config option (svg_extra_style) to add css rules after the default css rules rather than replacing the default css rules.

[caksoylar]

Thanks! Merged it at a1f3a18 and incorporated the isort changes into ea8484b. (It didn't let me push the unrelated history to your branch for some reason.)