feat: allow to set custom fonts

[caarlos0]

allows to set a custom font, e.g.:

invoice generate \
    --item "Yellow Rubber Duck" --quantity 5 \
    --item "Special Edition Plaid Rubber Duck" --quantity 1 \
    --note "For debugging purposes." \
    --fonts.regular.name BerkeleyMono \
    --fonts.regular.path ~/Downloads/berkeley-mono-typeface/berkeley-mono/TTF/BerkeleyMono-Regular.ttf \
    --fonts.bold.name BerkeleyMono-Bold \
    --fonts.bold.path ~/Downloads/berkeley-mono-typeface/berkeley-mono/TTF/BerkeleyMono-Bold.ttf \
    --output duck-invoice.pdf

would look like:

duck-invoice.pdf