Refactor netflow codec.

Netflow field definitions are now extracted to a bundled YAML file and also
introduces a new config directive to specify an additional YAML file that
can be used to augment the built-in list. This means adding new Netflow field
types doesn't require a rebuild of Logstash.

TODO Work out how to get the YAML file added to the jar.
Add example YAML to documentation
Pull Netflow YAML file into the jar.
Merge pull request #808 from bodgit/netflow

Break out Netflow v9 field definitions into YAML file