Introduce flat_list lookup plugin

note: main list or list (sub)items can also be set as a variable, which is the main idea for this plugin
``` yaml
- hosts: localhost
    gather_facts: False
    tasks:
    - debug: msg="{{item}}"
      with_flat_list:
      - a
      - - b
        - c
        - d
        - - e
        - f
        - - g
          - h
      - i
      - j
      - - k
        - l
```

  TASK: [debug msg="{{item}}"] **************************************************
  ok: [localhost] => (item=a) => {"item": "a", "msg": "a"}
  ok: [localhost] => (item=b) => {"item": "b", "msg": "b"}
  ok: [localhost] => (item=c) => {"item": "c", "msg": "c"}
  ok: [localhost] => (item=d) => {"item": "d", "msg": "d"}
  ok: [localhost] => (item=e) => {"item": "e", "msg": "e"}
  ok: [localhost] => (item=f) => {"item": "f", "msg": "f"}
  ok: [localhost] => (item=g) => {"item": "g", "msg": "g"}
  ok: [localhost] => (item=h) => {"item": "h", "msg": "h"}
  ok: [localhost] => (item=i) => {"item": "i", "msg": "i"}
  ok: [localhost] => (item=j) => {"item": "j", "msg": "j"}
  ok: [localhost] => (item=k) => {"item": "k", "msg": "k"}
  ok: [localhost] => (item=l) => {"item": "l", "msg": "l"}