Add support for color-scheme dependent pictures

[CosmicHorrorDev]

Resolves #231

Adds support for color-scheme dependent <picture>s like

<p align="center">
  <a href="https://nixos.org">
    <picture>
      <source media="(prefers-color-scheme: light)" srcset="https://raw.githubusercontent.com/NixOS/nixos-homepage/master/logo/nixos-hires.png"/>
      <source media="(prefers-color-scheme: dark)" srcset="https://raw.githubusercontent.com/NixOS/nixos-artwork/master/logo/nixos-white.png"/>
      <img src="https://raw.githubusercontent.com/NixOS/nixos-homepage/master/logo/nixos-hires.png" width="500px" alt="NixOS logo"/>
    </picture>
  </a>
</p>

which displays like so on this PR

Dark	Light

---

Implementation-wise the main parts involved

• Threading color_scheme: Option<ResolvedTheme> through everything to represent the None (default src), Some(Dark) (dark's srcset), and Some(Light) (light's srcset)
• Handling the <picture> meta-element which was done using a picture::Builder that stores relevant information from the various elements and finalizes on the closing tag. Overall I'm rather happy with this approach to complex elements