Only log controller parameters when configured to.

This adds a new config option `log_controller_parameters`, that defaults to false.  This needs to be set to true to enable logging of the params hash, otherwise this is omitted.

Rationale:

The change in shadabahmed/logstasher#14 has implications that are not immediately obvious.  It will cause the elasticsearch mapping for the parameters field to become a new dynamic mapping that gets indexed. This can lead to collisions if different apps (or even different controllers within the same app) use the same params key with different types, which results in log messages being lost.  It also causes the indexes to become significantly larger (in our case, this change tripled the size).

In light of this, I think the not logging these fields by default is the correct choice.