Use the newer { "fieldname" => "pattern" } for grok filters.

Replace the older grok style:

    { match => [ "fieldname", "pattern" ] }

With the newer one:

    { match => { "fieldname" => "pattern" } }

Also see https://github.com/elasticsearch/logstash/issues/1499
for more examples.

Closes #1516
Evaluate dynamic fields in event when used with gsub

Closes #1529