Make the emitter generic (Closes #162)

[szareiangm]

This is my first step towards improving elasticsearch loader. In this PR, I made the Emitter to receive genertic class type. My next step is to add filters (#165)

PS: Good sink can be handled separately from the emitter, mostly it is just Stdout. Also, There is a failing unit-test. I am checking why.

CC: @dmatth

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[szareiangm]

I signed it.