Added additional input arguments

A small PR to add arguments to phantomas_dwi script.

I found it useful to play with lambda 1,2,3 and to specify the output file name.

Maybe it would be better to use a JSON file to describe all parameter of the model (gm/wm/csf t1-t2-rho, gm/csf diffusivity, lambdas, te-tr...) What do you think?
I like this idea to add default parameters in the help messages! So that the solution be cleaner, you may wanna use that solution %(default)f proposed in StackOverflow: http://stackoverflow.com/a/18507871, rather than duplicating the value.