Defer tasks until a later named point.

[trulede]

This PR makes it possible to defer a command to a "named point" such as exit (i.e. when task itself is exiting) using a new when keywork. For example:

version: '3'
vars:
  GREETING: Hello, World!
tasks:
  default:
    cmds:
      - defer:
          task: when-message
          when: exit
      - defer:
          task: defer-message
      - task: message
  message:
    cmds:
      - echo "MESSAGE - {{.GREETING}}"
  defer-message:
    cmds:
      - echo "DEFER - {{.GREETING}}"
  when-message:
    run: once
    cmds:
      - echo "WHEN - {{.GREETING}}"

which produces this output (note the order of the messages):

$ go run ../cmd/task/task.go -s
MESSAGE - Hello, World!
DEFER - Hello, World!
WHEN - Hello, World!

The "when" mechanism operates in a method similar to a "pub/sub". Defered tasks a pushed to a named when list, and then pulled at some later point. Only the exit tasks are pulled and executed at the moment, however it would be possible to trigger the execution of tasks contained in a named list - such a mechanism only needs to be defined (e.g. a tempate function).

#2273
#2040
#1082
#141 - partial, would need addition, for instance defer/if (see #2345 (comment))

[trulede]

If this PR is interesting, I can develop further (tests, doc ... extensions for #141 ... etc).