matplotlib should be a declared dependency

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

```sh
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
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`.