Color palettes

[jbaa]

This pull request enables theme files to define a palette of named colors, and refer to them in the configuration values that assign colors to UI elements.

Working with a restricted set of colors facilitates theme creation. Additionally, the use of named color identifiers improves the readability of theme files.

The CL includes a theme file that demonstrates this feature by defining a theme using the gruvbox dark color palette.

[archseer]

This looks reasonable but it needs a bit of documenting in book/src/themes.md :)

[archseer]

I'll probably migrate over the default theme.toml after this is merged

[pickfire]

cc @DrZingo who may be better at reviewing this

I don't know how the choices behind the number of palettes.

[DrZingo]

A typo on purple0's color. The naming of 5 bg palette colors is consistent with the naming of gruvbox.

[pickfire]

Looks good to me.

[archseer]

Thanks! 🎉