Add support for "tailwind" function to look up config values

src/lib/evaluateTailwindFunctions.js

@@ -0,0 +1,12 @@
+import _ from 'lodash'
+import functions from 'postcss-functions'
+
+export default function(options) {
+  return functions({
+    functions: {
+      tailwind: function (path) {
+        return _.get(options, _.trim(path, `'"`))
+      }
+    }
+  })
+}