Add promisify method

lib/index.js

@@ -129,7 +129,7 @@ exports.getYYYYMMDD = function (date) {
 
 /**
  * sleep a while.
- * @param {Number} in milliseconds
+ * @param {Number} ms in milliseconds
  * @return {Promise} a Promise
  */
 exports.sleep = function (ms) {
@@ -138,6 +138,32 @@ exports.sleep = function (ms) {
   });
 };
 
+/**
+ * Wrap a standard async API to a promise-based API.
+ * ```js
+ * const readFile = kitx.promisify(fs.readFile);
+ * readFile(file).then((data) => {
+ *   // data
+ * }, (err) => {
+ *   // error
+ * });
+ * ```
+ * @param {Function} fn a strandard async API
+ * @return {Function} a Promise-based function
+ */
+exports.promisify = function(fn) {
+  return function (...args) {
+    return new Promise((resolve, reject) => {
+      fn(...args, function (err, data) {
+        if (err) {
+          return reject(err);
+        }
+        resolve(data);
+      });
+    });
+  };
+};
+
 /**
  * Get the IPv4 address
  * @return {String} the IPv4 address, or empty string