Allow sourcing zsh completion script (#1917)

Although it is not the recommended approach, sourcing a completion
script is the simplest way to get people to try using shell completion.
Not allowing it for zsh has turned out to complicate shell completion
adoption.  Further, many tools modify the zsh script to allow sourcing.

This commit allows sourcing of the zsh completion script.

Signed-off-by: Marc Khouzam <marc.khouzam@gmail.com>