Use shadow DOM for isolating `source_example` scope from external CSS

[robhrt7]

Right now, to isolate SourceJS UI styles from source_example blocks, we generate complex CSS selectors with Less. Would be great to replace this in future with Shadow DOM, as these guys are doing http://styleguide.sc5.io.

[robhrt7]

Maybe should be done first as a plugin before moving to core.

[robhrt7]

First experimental plugin version is now available https://github.com/sourcejs/sourcejs-shadow-dom.

Until decent browser-support, we can't use Shadow DOM in core.