Add support for font weight and type

[gwenwindflower]

I have very specific font specifications in Kitty terminal, it would be really cool to be able to use those same settings for the fonts. Right now if you choose a font family that only has Thin weights or Italics to try to force it to use that font, it just returns a default sans font.

Ideally I would like something like:

code_font_family = "EllographCF Nerd Font Thin"
watermark_font_family = "EllographCF Nerd Font Light Italic"

Love this plugin though, fantastic work!

[mistricky]

Great advice, I will add this feature in the next version