Make the footer more easily customisable

[welcome[bot]]

Welcome! Congrats on your first pull request to the Slate theme. If you haven't already, please be sure to check out the contributing guidelines.

[request-info[bot]]

Thanks for this. Do you mind providing a bit more information about what problem you're trying to solve?

[thefirethirteen]

woop pulled request to the wrong repo

my bad :)