Fix Vue compatibility (fix #790)

- Fix inability to instantiate reactive Vue components by 1) handling each child of #main instead of #main itself and 2)  skipping elements that are already Vue instances
- Retain previous behavior of processing basic Vue rendering without the need for a markdown <script> tag.
- Update documentation and add live Vue examples
- Update `index.html` files to include Vue.js and Vuep (CSS+JS)