feat(shortcode): add CSS class 'kofi' to make custom styling easier

layouts/partials/kofi_image.html

@@ -1,6 +1,6 @@
 <!-- Author: Tomasz Gągor https://github.com/tgagor/hugo-shortcode-kofi -->
 <!-- Ko-fi Image -->
-<a href='https://ko-fi.com/{{ .user }}' target='_blank'>
+<a class="kofi" href='https://ko-fi.com/{{ .user }}' target='_blank'>
     <img height='{{ .height }}' {{ if .width }}width='{{ .width }}' {{ end }} style='border:0px;height:{{ .height }}px;{{ if .width }}width:{{ .width }}px; {{ end }}'
         src='{{ .url }}'
         border='0' alt='{{ .alt_text }}' />