A commonmark viewer for egui

Example

# use egui_commonmark::*;
# use egui::__run_test_ui;
let markdown =
r"# Hello world

* A list
* [ ] Checkbox
";
// Stores image handles between each frame
let mut cache = CommonMarkCache::default();
# __run_test_ui(|ui| {
CommonMarkViewer::new("viewer").show(ui, &mut cache, markdown);
# });