Add "format" parameter to UDP input plugin.

Format parameter can be used to indicate how to parse the input data.
Allowed values are:
- **plain**: Do not try to parse the message received, provide the payload
    as `@message` attribute and fill the other attributes as before.
- **json_event** / **(undefined)** (default): Try to parse the payload as JSON,
    or fallback to plain text (as previous behavior).
- **json**: Try to parse the payload as JSON, filling the `@fields`
    attribute, or fallback to plain text.
- **json_syslog**: Try to parse the payload, starting at first '{', and
    filling the `@fields` attribute, or fallback to plain text
    (compatible with prefixed JSON message, such as for rfc3164 syslog).

The format parameter is meant to work similarly to http://logstash.net/docs/1.1.10/inputs/udp#setting_format and could be generalized to all input plugins.