Add source_url and issues_url to metadata
Make the antigen download URL parameterised and update the default.
Parameterise the git revision of the zgen repo.
This version alters the behaviour of the plugin the first time it's run.
Prior to this, if the user had a ~/.zshrc file, it would be added as a .zsh file to the ~/.zshrc.d/ directory, with order 00.
Starting from this version, the cookbook assumes all configuration the user requires is implemented as rcfile resources instead.