Add ZSH autocomplete compatibility

Backwards compatability in ZSH is described here
http://zshwiki.org/home/convert/bash. Most relevant is the line that
states ``To use it all you need to do is run bashcompinit at any time
after compinit. It will define complete and compgen functions
corresponding to the bash builtins.''

Tested with tig version 1.2.1 and ZSH version 5.0.2.

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>