add with_sequence lookup plugin

Plugin allows you to do easy counts for items.  Used as follows:

```yaml
----
- hosts: all

  tasks:

    # create groups
    - group: name=evens state=present

    - group: name=odds state=present

    # create 20 test users
    - user: name=$item state=present groups=odds
      with_sequence: testuser%02d 1 19 2

    - user: name=$item state=present groups=evens
      with_sequence: testuser%02d 2 20 2
```

You can optionally leave out the stride or format string.