Refer to FontAwesome icons by their class names

Forgive me if this has been asked and answered already, I couldn't find it in issues. It would be great for legibility if we could fetch icons by their class names, so `text=fa-plus` instead of `text=`.

They do exactly this at imgplaceholder.com:
https://imgplaceholder.com/400x400/ddd/fff/fa-plus?font-size=96

Start by locating where Holder processes the text option and how it currently handles character references. Compare the requested fa-plus class-name input with the existing numeric entity form and verify the resulting icon rendering. The issue names no files or tests, so the relevant entry point and regression coverage must be identified in the repository.

Written by the indexing model from the issue text.