Allow linking to external style sheet

[simonis]

Hi,

I think in some cases it may be useful to use CSS classes but not insert the styles into the generated document (because they get included elsewhere) or just link to an external style-sheet which is maintained outside of Rouge.

Exactly this can be achieved with this change with adds the new external option to the rouge-css attribute. This change shouldn't introduce any changes to the current functionality.

Best regards,
Volker

[gdamore]

Yes please!

[gdamore]

@jirutka this would make my life better -- in particular the EPUB formatter is very unhappy with <style> elements -- it causes EPUBs to fail validation, and some ereaders wind up ignoring the style or just putting it to the display as if it was body text (which is very, very bad).