Add basic background color generator

Supports specifying background colors as an array of colors, or an
object where the label is what you want to name that color as a
background utility, and the value is the color to use.

Everything can be specified as either camelCase or kebab-case so you
don't need to quote the shit out of everything but classes will always
be rendered as kebab-case.

src/generators/background-colors.js

@@ -0,0 +1,25 @@
+const postcss = require('postcss')
+const _ = require('lodash')
+
+function findColor(colors, color) {
+  return _.mapKeys(colors, (value, key) => {
+    return _.camelCase(key)
+  })[_.camelCase(color)]
+}
+
+module.exports = function ({ colors, backgroundColors }) {
+  if (_.isArray(backgroundColors)) {
+    backgroundColors = _(backgroundColors).map(color => [color, color]).fromPairs()
+  }
+
+  return _(backgroundColors).toPairs().map(([className, colorName]) => {
+    const kebabClass = _.kebabCase(className)
+
+    return postcss.rule({
+      selector: `.bg-${kebabClass}`
+    }).append({
+      prop: 'background-color',
+      value: findColor(colors, colorName)
+    })
+  }).value()
+}