Read file params

I needed the functionality to pass some parameters of pyclaw application scripts through
a parameter file instead of passing them directly on command line. I needed this when I used
the sumatra tool for experiments reproducibility.

This pull request contains my modification to the pyclaw util function _info_from_argv to enable
reading parameters file that has the format expected by json.load function, something like:
{"outdir" : "./Data",
 "use_petsc" : false,
 "sumatra_label" : null
}

The question is whether you think it will be nice to include this in pyclaw? or it will bring us back
to the idea of parameters files that was used in clawpack..
I'm using this file for the parameters that I would set in command line otherwise