Addition of highlight.js support through button in editor.
Updates to documentation in index.html

Highlight.js support merged from fork of bootstrap-wysihtml5 on github by evereq
https://github.com/evereq/bootstrap-wysihtml5
merge was taken from commit 518d40d57821413047b1560a7cb52c82a409fab7
and merged with head with minor updates from original code.