Add pillow (PIL) as dependecy

As requested by the owner, it is added to both requirements.txt and
setup.py - see also issue #172 about a possible redundancy/conflict.

requirements.txt

@@ -1,4 +1,5 @@
 .
 graphviz
+pillow
 pyyaml
 setuptools

setup.py

@@ -23,6 +23,7 @@ setup(
     long_description_content_type='text/markdown',
     install_requires=[
         'pyyaml',
+        'pillow',
         'graphviz',
         ],
     license='GPLv3',