Adding nested dictionary lookups

This lookup accepts nested dicts of potentially any depth, and recursively flattens them to a single dict with a `value` and one or more numbered keys (`key_0`, `key_1`, `key_N`).

My use case for this was dicts nested two levels deep for configuring an ini file (where the first level was a section name and the second level was the option name and the scalar value for it):
```yml
# roles/php/vars/main.yml
---
php_ini:
  PHP:
    short_open_tag: "Off"
    expose_php: "Off"
    error_reporting: "E_ALL & ~E_DEPRECATED & ~E_STRICT"
    display_errors: "Off"
    display_startup_errors: "Off"
    track_errors: "On"
    log_errors: "On"
    error_log: "/var/log/php_error.log"
  Date:
    "date.timezone": "America/Chicago"
  "mail function":
    sendmail_path: "sendmail -t -i"
  opcache:
    "opcache.enable": 1
    "opcache.enable_cli": 0
    "opcache.fast_shutdown": 0
```

And the example task:
```yml
# roles/php/tasks/main.yml
---
- name: Configure php.ini
  ini_file: >
    dest=/etc/php5/cli/php.ini
    section="{{ item.key_0 }}"
    option="{{ item.key_1 }}"
    value="{{ item.value }}"
  with_nested_dict: php_ini
```