resolves #115 add option to use font icons

- include Font Awesome in the default stylesheet
- introduce styles to draw each admonition icon w/ FontAwesome
- include caption in title of <i> element
resolves #115 add option for font-based icons

- attribute icons=font activates font-based icons
- font-based icon styles
- add toc2 class to body and #toc in asciidoc.conf compatibility file
- template cleanups
Merge pull request #299 from mojavelinux/admonition-font-icons

resolves #115 add option to use font icons