Introduces optional syntax highlighting

[LiquidityC]

Allows syntax highlighting to configured in the MadSkin by a caller
allow for custom syntax highlighting. Eg. through Syntext or a custom
function.

Fixes: #1

[Canop]

This isn't a trivial change, I'll have a look and review it but it won't be immediate.

[Canop]

Storing an optional reference to an highlighter in the skin looks right (the highlighter could probably be a trait, though).

I'm not thrilled by having the lang code carried by the FmtComposite and by having the FmtLine holding the already rendered line.
Is there a reason you didn't do the rendering when writing (i.e. in write_fmt_line) ?

[LiquidityC]

Thanks for taking a look at this!

On the code_lang field: I don't see a way to avoid it given the flat Vec<FmtLine> architecture. By the time highlighting runs, all CodeFence lines have been discarded, so the language tag has to be attached to the surviving code lines somehow. I can't see any obvious alternative here without introducing larger changes. Such as introducing a FmtLine::CodeBlock { lang: Option<String>, Vec<FmtComposite> }

On rendering in write_fmt_line instead of as a pre-pass: the obstacle is that most highlighters need the full block as context (multi-line strings, block comments, etc.), but write_fmt_line only sees one line at a time. Two paths I see:

• Line-by-line highlighting: simplifies the code significantly, but loses cross-line context — incorrect highlighting for anything spanning multiple lines. Eg. Comments.
• A FmtLine::CodeBlock variant that groups all lines of a block together with the language tag, letting write_fmt_line highlight the whole block at once. This also solves the code_lang-on-FmtComposite concern, but write_fmt_line would then emit a variable number of newlines per FmtLine, which changes its semantics.