Improve export as XHTML

The current copy as XHTML feature emits very, let's call it baroque, html. However, having a great export of code snippets highlighted in the colors and style of SubEthaEdit for blog posts or other media that takes in HTML would be great. Also to have some nice export options to include line numbers, styling etc. There are offerings out there that do this great, so it probably would be best to gather the desired functionality here in this issue first.

E.g. we could have a good semantic mapping from the scope strings to existing CSS style sheets from other source highlighters on the web. Have the option of line numbers in various ways, maybe folding etc.