Ability to send selected fields over the tcp socket

[niteshjain]

I have found this useful when the event needs to be sent via multiple output plugins. This allows me to send only certain fields to a particularity tcp destination.

I have tried to implement this and it can be viewed here

[niteshjain]

However after opening this issue, I realized that an event can also lots of fields and an inverse mechanism which would retain only the specified fields could also be useful.