Saving stdout and stderr to files

[steelstyle]

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

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

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