Formatter support

[nblumhardt]

From: https://nblumhardt.com/2016/07/serilog-2-0-json-improvements/#comment-2988455879

It appears that this package doesn't yet support the formatter arg necessary for configuring e.g. JSON formatting for a rolling file sink.

[skomis-mm]

Just checked with proposed config - works as expected on 2.1+. Need more info on this

[nblumhardt]

Confirmed working by the OP. (Thanks @skomis-mm )