Modify task-args to support pass-through-help meta.

By default, leiningen makes the following transformation:

	lein TASK_NAME --help → lein help TASK_NAME

For tasks that implement a help, this is inconvenient. This patch
disables this transformation for task vars and alias values marked with
the meta: :pass-through-help true
Don't require :pass-through-help to be a boolean.
Use a real lein task in pass-through-help tests, not funky var magic.
Dereference vector aliases in task-args.
Clarify :warn-on-reflection deprecation warning.
Add exclusions for cd-client.
Document :pass-through-help in plugin guide.