cssnext return a css string

Close #1

index.js

@@ -1 +1,20 @@
-console.log("@todo")
+/**
+ * Modules dependencies
+ */
+var postcss = require("postcss")
+
+/**
+ * Process a CSS `string`
+ *
+ * @param {String} string
+ * @param {Object} options
+ * @return {String}
+ */
+module.exports = function cssnext(string, options) {
+  // ensure options is an object
+  options = options || {}
+
+  return postcss()
+    .process(string, options)
+    .css
+}

test/index.js

@@ -1,7 +1,9 @@
 var test = require("tape")
 
+var cssnext = require("..")
+
 test("cssnext", function(t) {
-  t.equal(true, true, "cssnext is useless for now")
+  t.ok(typeof cssnext("html{}") === "string", "should return a string")
 
   t.end()
 })