[Suggestion] Add Effects to themes Palette or HighlightText

# Problem description
I'm creating my own theme. There I'd prefer to have highlighting less verbose.
Thus I'm trying to `Reverse` the highlighting colors, meaning the text itself gets highlighted (changes to the `Highlighting` color and the background is kept.)

# Code example
See the examples `SelectView` and `Theme*`

# Environment

* Operating system: Arch Linux
* Backend used: ncurses 6.1
* Current locale: de_DE.UTF-8
* Cursive version: 0.10

# Proposal
Easiest would be to change `ColorStyle::highlight()`s foreground to `Background` to allow a bit flexiblity.

Other more complex solutions could be allowing to introduce custom variants used then set or binding custom colors to them. (This sounds messy)

Another variant would be to just change them from `Color` -> `Colorstyle` in the palette.
This would break the current TOML-Theme-Syntax but streamlines Palette/ColorStyles.

You might have other/better ideas.

### PS
I'm a beginner at rust, so please excuse if I've overseen something completely obvious.