Add 'null' output which can be used to not send metrics

This output can be helpful when we want to use logster to just parse lines
in a log and act upon them in some way. This way, `get_state` function becomes
optional so there isn't a need to implement it which returns a `MetricObject`.

I'm not sure if it goes against the philosophy of logster as it's essentially for sending metrics but I needed something like logster which doesn't send metrics but act upon the log lines and does something. If there is a output present like `null`which doesn't call `submit_stats` function, that would be awesome. It just increases flexibility to the cases in which I can use logster.

May be, it's a bad idea, please feel free to close the PR if it seems bizarre.