Combine anchorlinenos and hl_lines for dynamic highlighting

[edbighead]

It is possible to highlight some lines in Highlight Shortcode using hl_lines parameter and anchorlinenos can be used to reference some specific lines of code.

While having a documentation website with code examples, it would be handy to reference some lines of code and have a highlight on them, like it's done on Github

[jmooring]

I think this needs to happen first: alecthomas/chroma#373.

Additionally, it would be easier to identify ranges of line numbers in the URL fragment if a hyphen was not appended to the string specified in lineanchors. If you have two code blocks on a page, and you set lineanchors for each block as A and B respectively, the fragment to specify lines 2-4 would look like #B-2-4 or #B-2-B-4. In GitHub it looks like #L4-L7 which is simpler to parse.