Add --font flag

[mateosss]

Add a --font or -f flag to ./set-matter.sh with three options poiret-one, pattaya, and karla.
You can get the ttf files for those fonts here.

The idea is that the ttf files will be inside a fonts folder, and based on the selection, grub-mkfont will be used to generate the pf2 file and set the corresponding font in theme.txt.

[satooru]

I created a pull request regarding this issue, can you check it?

[mateosss]

Done in the new python update.