Allow screenshot difference percentage to be configured

[thomasjbradley]

Maybe add a new property to the screenshot tests like:

screenshots:
  differenceOffset: +7

Positive numbers to make it easier.

screenshots:
  differenceOffset: -4

Or negative numbers to make it harder.

Would add or subtract that number from the default matching percentage.

[thomasjbradley]

Would be helpful for the following assignments:

• https://github.com/acgd-webdev-3/modularity-mindfulness (Easier 20%)
• https://github.com/acgd-webdev-3/this-site-aint-no-sloth (Easier)
• https://github.com/acgd-webdev-1/call-to-action-banner (Harder)
• https://github.com/acgd-webdev-1/hero-banner (Harder)