Budspencer theme

I tried to translate my [prezto budspencer theme][prezto-budspencer] to fish syntax. There are still some things missing. Especially the elapsed time segment cannot be implemented until a `preexec`/`postexec` command is implemented in fish. However, the theme should be completely usable how it is. I am looking for volunteers to test it carefully and send me bug reports. All the nice features are documented in the README file.

You can merge it, if you like it.

If anybody knows how to change the cursor color like in the original [prezto theme][prezto-budspencer], please tell me!
[prezto-budspencer]: https://github.com/tannhuber/prezto
You might want to check this [comment](https://github.com/fish-shell/fish-shell/issues/1583#issuecomment-50301797)