Add Netflow input plugin, currently supporting v5:

- A log event is created for each flow record in a received packet. @fields
  contains all of the fields in the flow records plus some of the ones found
  in the global header.

- @timestamp is taken from the epoch seconds/nanoseconds provided in the
  flowset header.

- IP address fields are converted from uint32 to dotted quad.

- First/Last switched records are converted from device uptime milliseconds
  into a UTC timestamp.