Support Elasticsearch Auth

[Foolius]

I need stream2es to support the ES Auth plugin ( https://github.com/codelibs/elasticsearch-auth ) and want to program this myself, but I'm not sure how I should do it.
I think one possibility would be to add username and password parameters.

Another more generic solution is to add only one parameter and with this parameter one can expand the json payload.

For example, when you add the option --additional "token":"uineqlxvonrtueaiobud" the json which gets send looks like this: {[old json], "token":"uineqlxvonrtueaiobud"}

Which one of these solutions do you prefer?