Saving stdout and stderr to files

I wold like to know what is the recommanded way to save stdout and stderr to files using gaffer, also it seem that using Procfile I can't read the stream neither.

It might be due to the hardcoded value here :
[gaffer/cli/commands/load.py:100](https://github.com/benoitc/gaffer/blob/1e12a83c62e8f4f54256df41ed84b87648a5ff97/gaffer/cli/commands/load.py#L100)

So here again I suggest to add a new argument to override the default value.

Ps: I can work on fixes/features if needed.