Added ability to randomise the seed of each chain

For some particular cognitive modelling task I have been working on, I've needed to be able to randomise the seeds so that calls to matjags.m are not identical due to setting the seed as equal to the chain number.

I've implemented these changes (just a few lines) and then added it as an optional input argument. I've tested it a little and it seems backwards compatible. If you do not specify the new 'rndseed' input flag then the default rndseed=0 is used.