Add proc macros for compile time parsing

[lampsitter]

Closes #49

Example

commonmark!("n1", ui, &mut self.cache, "# Hello, world");

Generates the following

    {
        let ui: &mut egui::Ui = ui;
        ::egui_commonmark_backend::prepare_show(&mut self.cache, ui.ctx());
        let options = ::egui_commonmark_backend::CommonMarkOptions::default();
        let max_width = options.max_width(ui);
        let layout = egui::Layout::left_to_right(egui::Align::BOTTOM).with_main_wrap(true);
        ui.allocate_ui_with_layout(egui::vec2(max_width, 0.0), layout, |ui| {
            ui.spacing_mut().item_spacing.x = 0.0;
            let height = ui.text_style_height(&egui::TextStyle::Body);
            ui.set_row_height(height);
            ::egui_commonmark_backend::newline(ui);
            ui.label(egui::RichText::new("Hello, world").strong().heading());
            ::egui_commonmark_backend::newline(ui);
        })
    }