matplotlib should be a declared dependency

[bergercookie]

Currently after a fresh installation of the tool, and after using the --plot flag, I get the following error:

psrecord 15320 --log output.log --plot graph.png
Attaching to process 15320
^CTraceback (most recent call last):
  File "/home/berger/.local/bin/psrecord", line 7, in <module>
    sys.exit(psrecord.main())
  File "/home/berger/.local/lib/python3.6/site-packages/psrecord/main.py", line 104, in main
    interval=args.interval, include_children=args.include_children)
  File "/home/berger/.local/lib/python3.6/site-packages/psrecord/main.py", line 209, in monitor
    import matplotlib.pyplot as plt

I think this is because matplotlib is not a declared dependency in setup.py.