#3 Implement Zsh support

[Tetsujin]

So far the library has targeted two shells: Korn Shell and Bash. Support for Zsh has been planned but so far not completed, in part due to some difficulties in understanding how to adapt the code and build system to Zsh. Solutions should be found and implemented,

Acceptance Criteria
Zsh should be available as a Shell-Pepper build target: All current Shell-Pepper functionality should be provided in the Zsh version, and going forward the Zsh target should be kept up-to-date with new functionality as it is introduced