Add Fish-like theme

[Alunderin]

I quite like Fish's default prompt, so I was a bit miffed that the only similar ones in ZSH/Prezto didn't quite meet my standards, e.g. no status reporting, no truncated paths, colors I'm not a fan of.

So here's my take on a proper Fish theme:

The only part I'm less certain about is setting PROMPT_EOL_MARK. I don't like the default and this seems like an appropriate place to change it. No other themes are doing this however.

[Alunderin]

Okay, so I decided to finally look at closed PRs related to themes just to be meet with a sea of red. Apparently, Prezto core doesn't accept new themes? I should instead try to contribute this to prezto-contrib, but

1. I didn't even know this repo existed until stumbling across this closed PR. If you want people to know about it, it should be present in the README.
2. How should I have known that this project doesn't accept new themes? The contribution guidlines never alludes to this.

And sorry for making such a big deal out of this one comment made by a contributor. I just feel a little mislead when I try to do things correctly and still end up creating something that's dead in the water before I even started.