More flexible destinations for file writer

[emlyn]

It would be useful to allow writing logs to other places than a local file. This change allows it to accept anything that can be coerced into a writer (such as e.g. a Hadoop FSDataOutputStream for writing to a remote filesystem). The current behaviour of creating the parent directories is maintained if the filename passed in is a string (or a File object).

[emlyn]

I realise that filename isn't necessarily the best name for this config after this change; ideally it would be something more generic (output-location? destination?), but I'm not sure it warrants breaking compatibility.

[BrunoBonacci]

thank you!