Better support for a 'flat' output.

[jakelandis]

Currently when this plugin is used, the output can be very hierarchical with objects of objects.

For example:

 "element" => [
            [ 0] {
                 "name" => "articleid",
                "value" => [
                    [0] "478879"
                ]
            }

Dealing with deep nested structures of arrays may not always be easy (in elasticsearch or logstash).

It may be that the desired output is a flat output like:

 "articleid" => "478879"

Other filters such as mutate can be used to move this data to get the desired output, however via some configuration (and new code!) Logstash should be able to provide an easier experience to flatten out these types of data structures.