Add North gravity for imagemagick

[domdoescode]

Doing this to allow loltext to be centred at the top of the image.

As listed in the imagemagick CLI docs - https://www.imagemagick.org/script/command-line-options.php#gravity

[matthutchinson]

Thanks for fixing 👍 No idea why i'd forgotten to add North 🙃

[domdoescode]

😁