Support rotation when tracing to new logfile

lager:trace_file/3 can now also handle a new-style configuration proplist
to allow full configuration of a newly created file backend.
If the file backend already exists, however, only the level can be given
(similarly to the old behaviour).

I initially created this commit weeks ago. It would be nice to have persistent tracing into file fully configurable (and I am ready to add this to this branch) but I am not sure how the syntax should look like. (My concern is the handling of Level: should it be part of the backend config or is it a separate mandatory argument of the tracer)