Add ability to use spot colors

some commercial printers use spot colors ex. `PANTONE 534 EC` generally you specify the tint percentage as well when using them. i know mpdf has the `AddSpotColor` option and a couple others as well i believe. you can specify spot colors in css as well which would probably be needed for the headers and places background colors are used. Ex. ``

https://mpdf.github.io/reference/mpdf-functions/addspotcolor.html