Add basic zsh completion script

Add a basic zsh completion script for `git-hub` and `git hub`, at
`share/zsh-completion/_git-hub`, that, if hooked into the zsh completion
system, simply causes zsh to complete available `git-hub` subcommands
(as listed in `lib/git-hub.d/completion`).
`lib/git-hub.d/init`: Set up zsh compl when in zsh

In `lib/git-hub.d/init`, when executing under zsh, prepend the path
`$GIT_HUB_ROOT/share/zsh-completion` to the zsh array parameter `fpath`, so
that our zsh completion script, which is in `share/zsh-completion`, will be
hooked into the zsh completion system.
Merge branch '8573/zsh-completion/basic/1' into install

Merge the personal topic branch `8573/zsh-completion/basic/1` into the
`install` branch.

Branch `8573/zsh-completion/basic/1` includes:

 - a zsh completion script, at `share/zsh-completion/_git-hub`; and

 - shell environment initialization code, in `lib/git-hub.d/init`, to
   set up the shell environment, when in zsh, so as to utilize the above
   zsh completion script.