[Feature] command completion for zsh

[santosh]

As soon as rez installation is finished, the following message is shown:

You may also want to source the completion script (for bash):
source /home/sntshk/rez/completion/complete.sh

That directory contains 2 files, for bash and for csh.

I'm not quite sure how autocompletion is handled in zsh (autocompletion is the selling point of zsh). But there must be some way we can autocomplete package name after rez env?

Note: This issue is also related to #451.

[maxnbk]

I have a WIP zsh plugin attempt I plan to PR soon.

[nerdvegas]

Fixed by #711