Better formatting options

Hi. Your plugin is great. However I have a small suggestion to allow for much better styling of the radio buttons. I edited the plugin files at line 105 of wc-variations-radio-buttons.php from:

printf( '<div><input type="radio" name="%1$s" value="%2$s" id="%3$s" %4$s><label for="%3$s">%5$s</label></div>', $input_name, $esc_value, $id, $checked, $filtered_label );

to:

printf( '<div><input type="radio" name="%1$s" value="%2$s" id="%3$s" %4$s><label for="%3$s">%5$s</label><span class="checkmark"></span></div>', $input_name, $esc_value, $id, $checked, $filtered_label );

I could then use pure CSS to make pretty radio buttons that changed appearance on selection. Eg like outlined at https://www.w3schools.com/howto/howto_css_custom_checkbox.asp

It would be great if you could include that in the plugin in the future.