add Promise#finally

fix then#78

Author: edef1c

index.js

@@ -2,5 +2,6 @@
 
 module.exports = require('./lib/core.js')
 require('./lib/done.js')
+require('./lib/finally.js')
 require('./lib/es6-extensions.js')
-require('./lib/node-extensions.js')
+require('./lib/node-extensions.js')

lib/finally.js

@@ -0,0 +1,16 @@
+'use strict';
+
+var Promise = require('./core.js')
+
+module.exports = Promise
+Promise.prototype['finally'] = function (f) {
+  return this.then(function (value) {
+    return Promise.resolve(f()).then(function () {
+      return value
+    })
+  }, function (err) {
+    return Promise.resolve(f()).then(function () {
+      throw err
+    })
+  })
+}