Syntax highlighting using highlight.js

Addition of button to wrap code examples in <code><pre></pre></code> tags and have syntax highlighting applied via highlight.js

Done through merge of code from another fork of bootstrap-wysihtml5 maintained by evereq.
https://github.com/evereq/bootstrap-wysihtml5

merge was taken from commit 518d40d57821413047b1560a7cb52c82a409fab7