Use font-based icons for admonitions

This change causes NOTE, TIP, and other admonitions to be rendered with
pretty icons instead of plain text.

See:
 - https://asciidoctor.org/docs/user-manual/#admonition
 - https://asciidoctor.org/docs/user-manual/#admonition-icons

build.gradle

@@ -13,6 +13,7 @@ asciidoctor {
     }
 
     attributes 'source-highlighter': 'coderay',
+        icons               : 'font',
         idprefix            : '',
         idseparator         : '-',
         docinfo1            : ''