Allow editing of log_record in two possible ways

One more thing related to https://github.com/madzak/python-json-logger/issues/16: I just noticed that while `process_log_record` returns the `log_record`, that returned value isn't actually stored in a variable. By making the change in this commit, the user can use two approaches in `process_log_record` in a subclass to modify the `log_record`:

- modify the given `log_record` in `process_log_record` (in-place) and return that,
- or, use the given `log_record` to compute an entirely new dictionary instance and return that instead.