Add option to copy (colored) document body only

Add an option that allows to export colored source code
- using CSS
- but only export the <pre>...</pre> element
- without a complete document

This makes sense in an environment where the CSS is already present as
an external style sheet (e.g. in the Lilypond blog ...)
Merge pull request #397 from uliska/copy-body-only

Add option to copy (colored) document body only