Support additional themes in config

[tmandry]

This removes the need for replacing all of index.hbs just to have a custom theme.

Closes #2048.

[expikr]

Very nice, can you also copy over my Orange example to the test book as well, so that the PR can be fully superceded?

[ehuss]

Some drive-by comments:

One issue we've had is that the id generated for the theme selector can conflict with id's used in headers and other places. For example, you can't have a header like ## Rust because it conflicts with the Rust color scheme. I don't know how we can better handle that, and I worry a little that supporting additional themes will make it harder to fix that (or make collisions more likely). That's probably not a blocker, but I figured I'd bring it up since I worry about it.

I'd also like to understand a little more how other generators work. #2048 discusses that a little, but I haven't had time to read and understand. I'm just trying to understand what the tradeoffs or alternate designs are here.

I also wonder what if the user wants to remove a stock theme, how do they do that?

I assume this works with all the options in output.html like default-theme and preferred-dark-theme?

I've been slightly uncomfortable that there is some muddled terminology here. "theme" in mdbook is a directory that allows you to completely customize how a page is rendered. Then there is also a "color theme" which is just a css class and a selector. The two are only loosely releated, and I worry about the potential confusion around them. I don't know why it was structured that way.

Otherwise seems like it would be fun to support this.