Add custom scheme combinations

[mattxwang]

Right now, this PR is just a sketch of the feature described in #483. It does work!

To test:

• first, run site as-is in the branch; observe that there are no errors (backwards-compatible)
• add a custom scheme
  • create a custom_schemes folder
  • place a file in it (ex boop.scss)
  • add an overriding style (ex * { color: purple !important }
  • in _config.yml, define custom_schemes: ["boop"]
  • reload jekyll serve
  • observe that the site is now purple :)
• add multiple schemes
  • rinse and repeat from above!

Things to still resolve:

• where in the import order should this go? went with the original suggestion in the issue
• documentation (if we like this implementation, I'll write some up)
• in the short term, do we want to restructure any of our scheme/theme code to use this new key? likely not, right?

Closes #483.

[mattxwang]

@Ninja-mann07 it seems like you may have posted a patch by accident?