Support boolean flag to retain HTTP headers

[jmkgreen]

Looks like there are a few fields recorded by this plugin that we have no desire to keep in our storage and will therefore have to write filters to to discard them.

It would be nice to see a flag named retain_http_headers which could be set to true by default for backward compatibility reasons. When false these headers are simply not populated or are automatically filtered out.

[magnusbaeck]

I suggest the fields are stored under @metadata, similar to how the rabbitmq input deals with message metadata. This is actually also suggested in #10.

[ebuildy]

#68 should do the trick

[jsvd]

solved by #68