grim/convey

Remove the convey/extend task as it's normal usage is already covered by plan/stage environment variables. It's corner case, expanding variables, is rather exotic and not something that comes up often.
tasks:
slow:
type: docker/run
image: alpine:edge
script: sleep 5
plans:
default:
stages:
- name: first
tasks: slow
- name: second
tasks: slow
- name: third
tasks: slow