resolves #264 add line highlighting option

[mojavelinux]

• add line highlighting option to HTMLLinewise formatter
• add dedicated formatter to add line highlighting (only marks highlighted lines)

[pyrmont]

Sorry about the delay, @mojavelinux. I got a bit side-tracked with various things.

@jneen has expressed in the past a reluctance to add further formatters to Rouge. @jneen, what do you think about this PR?

[mojavelinux]

Thanks. As I said in #264, "If you think the additional formatter is unnecessary, I can remove it."

[jneen]

This seems okay to me, mostly because it's its own class. My major reluctance is to adding options to existing formatters.

[mojavelinux]

Thanks for reviewing. I've gone ahead and removed the change that introduced the additional options to the existing formatter. Let me know if you need me to make any other changes.

[nektro]

Any updates on this / is there anything blocking?

[jneen]

Looks good to me. It's using a \n for a newline, which seems to indicate it needs to be in a <pre> block of some kind, rather than using divs or otherwise. But I think it's okay. There's so many ways to format in HTML that I consider the builtin HTML formatters here to be more like... documentation. Or just the very basic use cases. I'm +1 to merge.

[jneen]

and merged! thanks all <3