Add support for highlighting source code fragments

[javier-godoy]

Add support for highlighting source code fragments in order to emphasize a section of the code snippet. A block is delimited by // begin-block name and // end-block comments. Nested blocks are not supported.

 // begin-block first
Div first = new Div(new Text("First"));
SourceCodeViewer.highlightOnHover(first, "first");
add(first);
// end-block

[paodb]

I saw it in action and I think this is a great improvement!