Images are not optional

[jacobtomlinson]

First I want to say awesome project!

I'm trying to build a chart repo which currently doesn't have any images. The README says images are optional however I am unable to get it to run without them.

If I omit imagePrefix and images I get:

Traceback (most recent call last):
  File "/opt/boxen/homebrew/bin/chartpress", line 11, in <module>
    load_entry_point('chartpress==0.2.0.dev0', 'console_scripts', 'chartpress')()
  File "/opt/boxen/homebrew/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chartpress.py", line 204, in main
    prefix=chart['imagePrefix'],
  File "/opt/boxen/homebrew/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/ruamel/yaml/comments.py", line 702, in __getitem__
    return ordereddict.__getitem__(self, key)
KeyError: 'imagePrefix'

If I set imagePrefix then I get:

Traceback (most recent call last):
  File "/opt/boxen/homebrew/bin/chartpress", line 11, in <module>
    load_entry_point('chartpress==0.2.0.dev0', 'console_scripts', 'chartpress')()
  File "/opt/boxen/homebrew/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chartpress.py", line 205, in main
    images=chart['images'],
  File "/opt/boxen/homebrew/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/ruamel/yaml/comments.py", line 702, in __getitem__
    return ordereddict.__getitem__(self, key)
KeyError: 'images'

If I set images to None I get:

Traceback (most recent call last):
  File "/opt/boxen/homebrew/bin/chartpress", line 11, in <module>
    load_entry_point('chartpress==0.2.0.dev0', 'console_scripts', 'chartpress')()
  File "/opt/boxen/homebrew/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chartpress.py", line 208, in main
    push=args.push,
  File "/opt/boxen/homebrew/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chartpress.py", line 65, in build_images
    for name, options in images.items():
AttributeError: 'NoneType' object has no attribute 'items'

If I set images to an empty list (images: []) I get:

Traceback (most recent call last):
  File "/opt/boxen/homebrew/bin/chartpress", line 11, in <module>
    load_entry_point('chartpress==0.2.0.dev0', 'console_scripts', 'chartpress')()
  File "/opt/boxen/homebrew/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chartpress.py", line 208, in main
    push=args.push,
  File "/opt/boxen/homebrew/Cellar/python3/3.6.3/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chartpress.py", line 65, in build_images
    for name, options in images.items():
AttributeError: 'CommentedSeq' object has no attribute 'items'