Add support for mixed "plain color string" and "color map" background defintitions

Lets you do this:

```
  backgroundColors: [
    'blue',
    'purple',
    'pink',
    {
      primary: 'white',
      secondary: 'orange',
    },
  ]
```

src/generators/background-colors.js

@@ -11,7 +11,12 @@ function findColor(colors, color) {
 
 module.exports = function ({ colors, backgroundColors }) {
   if (_.isArray(backgroundColors)) {
-    backgroundColors = _(backgroundColors).map(color => [color, color]).fromPairs()
+    backgroundColors = _(backgroundColors).flatMap(color => {
+      if (_.isString(color)) {
+        return [[color, color]]
+      }
+      return _.toPairs(color)
+    }).fromPairs()
   }
 
   return _(backgroundColors).toPairs().map(([className, colorName]) => {