Completion Script - zsh

- With P.C. Shyamshankar's blessing, updated the _task script for zsh
  by using the new task built-in _commands command to enumerate all
  supported commands.  This replaces code that forces task usage output,
  and parses it for commands.
- Note that _task for zsh lacks attribute completion.  We need a zsh
  developer to make this enhancement.

scripts/zsh/_task

@@ -27,10 +27,7 @@
 #
 
 typeset -g _task_cmds
-_task_cmds=($(task rubbish-command | sed -n -e 's/^\s\+task \(\w\+\) .*/\1/p' | grep -v ID))
-
-# As of task 1.7.0, 
-# _task_cmds=(add append annotate completed edit duplicate delete undelete info start stop done undo projects tags summary timesheet history ghistory next calendar active overdue stats import export color version help list long ls newest oldest)
+_task_cmds=($(task _commands))
 
 _task() {
     _arguments -s -S \