Add option to wrap long lines in code blocks

[cauebs]

Horizontal scrolling is not very good. Maybe this could be a setting, maybe there could be a button besides the "copy to clipboard" one. What are your thoughts?

[sanmai-NL]

See highlightjs/highlight.js#881 for a workaround with custom CSS, perhaps a key element of an initial implementation.

[astr0n8t]

You can do this by creating a custom.css file next to your book.toml with the following:

pre, code {
    white-space: pre-wrap;
}

And then adding the following to your book.toml like the docs say:

[output.html]
additional-css = ["custom.css"]