Expand scope of color-scheme feature

[foolip]

@lilles has brought to my attention that <meta name="color-scheme" content="dark light"> shipped at the same time at the color-scheme CSS property in at least Chrome:
https://chromestatus.com/feature/6070987093180416
https://chromestatus.com/feature/5330651267989504

<meta name="color-scheme"> affects what color-scheme:normal means and @lilles thinks it's plausible that the meta tag is more widely used than the CSS property.

I would just send a PR for this, but #1551 also concerns color-scheme, so I think it's best to discuss both and make a single change instead of trickling the changes.