Allow Custom Fonts in RedReader

[Tanfj]

It has font scaling which is a help, but there are custom fonts designed for those with limited vision and other learning disabilities.

This would be a useful addition for it's accessibility and features vs other products.

[nascentt]

This is a duplicate of https://github.com/QuantumBadger/RedReader/issues/1087

It's something that's been requested for years via the subreddit and makes sense for accessibility reasons