Allow dynamic field templating in "copy" operation

[tsouza]

The following does not works:

mutate {
    copy => ["SomeField", "Other%{FieldName}"]
}

I guess we could fix this by adding the following:

dest_field = event.sprintf(dest_field)

right before line https://github.com/logstash-plugins/logstash-filter-mutate/blob/master/lib/logstash/filters/mutate.rb#L456