Implement configuration panel to change default visual library settings

[D1360-64RC14]

Each used library have it's own settings, that can't¹ be changed currently.

Mermaid drove me to write this issue, but, looking at all the libraries, these are the settings I see as useful to change, for now:

• highlight.js allows changing the theme;
• MathJax allow changing the math font;
• Mermaid allows changing the theme and look (drawing style);
• JoyPixels allows changing skin tone.

Footnotes

1. *Mermaid allows changing some configs per chart. ↩