Changing the target of logging #369

We have an application that runs in multiple batches where inputs and outputs of a given batch have their own directory batchX. It would be useful if the log file were also inside this directory which requires changing the target at runtime. We currently do this by defining a custom type with internal locking to impl Write on and then initialize it as static to satisfy 'static lifetime of Target::Pipe, which seems a bit awkward.

Is there a better way to accommodate this?