[zsh] Total rework of zsh files.

Thanks to lokaltog for providing me with a really good idea of what
proper zshrc files are supposed to look like.

- Move zsh configuration over from several small files into zshrc.

- Cleans up the repository as a whole. Remove several no longer needed
  files.

- Adds setopt section.

- New better, more readable and correct prompt.

- Correct spelling and formatting errors in README.md

- A lot of minor fixes and improvements.

- Updated tmux.conf