Feature request: Add function adding licencing information for icons used

[maelle]

See e.g. https://fontawesome.com/license

The function I have in mind would look for all icon calls in the RMarkdown and output text corresponding to licence of the sources. Then if I say make a poster with RMarkdown and icon, I'd call this function in a chunk to have legal stuff at the bottom.

Thanks @stephlocke for the idea.

[mitchelloharawild]

Re-opening as it was incorrectly auto-closed from rebasing commits from the alternate repo.