Support inlining CSS for a performance boost.

[adworacz]

Can reduce First Content Paint time and overall download size for a single page. This is useful for sites where often only one page is visited by a user. In such a case, separately loading/caching the CSS provides no extra benefit, and slows down content rendering while waiting the CSS network request to start and finish.

Additionally, when using a decent compression algorithm like Brotli, the total number of bytes sent over the wire can drop a bit. In my testing, I saw total CSS + HTML size drop by 0.17KB, or ~2.5%.

Original idea taken from "Lightspeed" Zola theme: https://github.com/carpetscheme/lightspeed/blob/master/templates/index.html#L15-L16

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[ejmg]

whoops, looks like merging #20 gave a conflict. if you get the chance to resolve it, we're good to close up your final outstanding PR!

[adworacz]

Sounds good - will fix this conflict in the next few days.