Ability to specify a plot save directory

PR here: #56, example usage here: https://github.com/kundajelab/tfmodisco/blob/682faf6ef2dc40bbf4f3b0fdd57a23000e8737a1/test/test_tfmodisco_workflow.py#L117. Helps to avoid the issue of seqlet score distribution plots getting overridden when multiple tf-modisco jobs are launched from the same directory.