[FEATURE] Impl `serde::Deserialize` for theming types that support loading from TOML

**Is your feature request related to a problem? Please describe.**
I am making an application that has some text that uses colors that come from outside of the palette. I want to support fully customizing the theme, and for everything but that text, I have it load a cursive theme toml file. I've added a section to the main application configuration that loads the remaining colors, but to use serde, I had to wrap the `Color` type in my own type to implement the traits, wrapping the existing `parse` method.

**Describe the solution you'd like**
Since cursive already depends on serde, it would be nice if the serde traits were implemented, so that it would be easier to load cursive types from custom toml configuration files.

**Describe alternatives you've considered**
I didn't see this originally, but #253 would also be a great solution, and would let me keep all the theming in one file instead of split between the two.

**Additional context**
Add any other context or screenshots about the feature request here.