Standalone generation script

[BastiPaeltz]

This adds a standalone script so sourcing the plugin to zsh is not required, which saves loading time on shell startup.
Generating completions with the zsh-completion-generator is rather something you do once in a while and does not need to be sourced every time you start zsh in my opinion.
Also made the script a bit more verbose (help message, errors) and added support for custom help commands.

EDIT:
In addition one could add the following to the usage message to be super clear how to get the output into working zsh completions.

 zshgencomp ack > _ack
 mv _ack /usr/local/share/zsh/site-functions/

[RobSis]

Well, the plugin is running the python script only once, so I'm not sure how a big problem the loading time is. Is it that bad? Should we create ticket for a better performance?

Anyway, I don't like having two nearly identical scripts in the repo. Can the new script be used from the plugin? If yes, then lets drop the old one altogether...