New: Add graphic and background image options (fix #114)

[swashbuck]

Fix #114

New

• Adds _graphic option for an image (ex. a logo) that appears before the title.
• Adds _backgroundImage and _backgroundStyles options for a page background image

Update

• markdownlint fixes to README.md

Testing

Add new properties to _languagePicker object in config.json:

"_graphic": {
  "src": "course/en/images/logo.png",
  "alt": "Logo"
},
"_backgroundImage": {
  "_xlarge": "course/en/images/background.jpg",
  "_large": "course/en/images/background.jpg",
  "_medium": "course/en/images/background-sm.jpg",
  "_small": "course/en/images/background-sm.jpg"
},
"_backgroundStyles": {
  "_backgroundSize": "contain",
  "_backgroundRepeat": "repeat",
  "_backgroundPosition": "center center"
},

Screenshots

Migration scripts 🚧

• Need to add migration scripts. Migration scripts PR is currently open Update: added migration scripts (fixes #103) #104 . So, if this PR is approved, either add to that PR or (if it's been approved) add new migration scripts to this one.

[swashbuck]

Ready for review

[cahirodoherty-learningpool]

👀

[joe-allen-89]

👍

[kirsty-hames]

Works as expected thanks @swashbuck 👍 I've just raised some minor comments