Add test to make sure custom config is actually used

__tests__/customConfig.test.js

@@ -0,0 +1,28 @@
+import path from 'path'
+import postcss from 'postcss'
+import tailwind from '../src/index'
+
+test('it uses the values from the custom config file', () => {
+  return postcss([tailwind(path.resolve(`${__dirname}/fixtures/customConfig.js`))])
+    .process(`
+      @responsive {
+        .foo {
+          color: blue;
+        }
+      }
+    `)
+    .then(result => {
+      const expected = `
+        .foo {
+          color: blue;
+        }
+        @media (min-width: 400px) {
+          .mobile\\:foo {
+            color: blue;
+          }
+        }
+      `
+
+      expect(result.css).toMatchCss(expected)
+    })
+})

__tests__/fixtures/customConfig.js

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+module.exports = {
+    screens: {
+        mobile: '400px',
+    }
+}