feat: gettext from app (#38)

```js
app.__(locale, text, value);
```

README.md

@@ -76,9 +76,9 @@ locales({
 Get current request locale text.
 
 ```js
-function* home() {
-  this.body = {
-    message: this.__('Hello, %s', 'fengmk2'),
+async function home(ctx) {
+  ctx.body = {
+    message: ctx.__('Hello, %s', 'fengmk2'),
   };
 }
 ```
@@ -111,6 +111,15 @@ Set locale and cookie.
 
 Where does locale come from, could be `query`, `cookie`, `header` and `default`.
 
+### `app.__(locale, key[, value1[, value2, ...]])`
+
+Get the given locale text on application level.
+
+```js
+console.log(app.__('zh', 'Hello'));
+// stdout '你好' for Chinese
+```
+
 ## Usage on template
 
 ```js

index.js

@@ -71,13 +71,17 @@ module.exports = function (app, options) {
 
   debug('Init locales with %j, got %j resources', options, Object.keys(resources));
 
-  app.context[functionName] = function (key, value) {
-    if (arguments.length === 0) {
+  if (typeof app[functionName] !== 'undefined') {
+    console.warn('[koa-locales] will override exists "%s" function on app', functionName);
+  }
+
+  function gettext(locale, key, value) {
+    if (arguments.length === 0 || arguments.length === 1) {
       // __()
+      // --('en')
       return '';
     }
 
-    const locale = this.__getLocale();
     const resource = resources[locale] || {};
 
     let text = resource[key];
@@ -90,38 +94,61 @@ module.exports = function (app, options) {
       return '';
     }
 
-    if (arguments.length === 1) {
-      // __(key)
+    if (arguments.length === 2) {
+      // __(locale, key)
       return text;
     }
-    if (arguments.length === 2) {
+    if (arguments.length === 3) {
       if (isObject(value)) {
-        // __(key, object)
-        // __('{a} {b} {b} {a}', {a: 'foo', b: 'bar'})
+        // __(locale, key, object)
+        // __('zh', '{a} {b} {b} {a}', {a: 'foo', b: 'bar'})
         // =>
         // foo bar bar foo
         return formatWithObject(text, value);
       }
 
       if (Array.isArray(value)) {
-        // __(key, array)
-        // __('{0} {1} {1} {0}', ['foo', 'bar'])
+        // __(locale, key, array)
+        // __('zh', '{0} {1} {1} {0}', ['foo', 'bar'])
         // =>
         // foo bar bar foo
         return formatWithArray(text, value);
       }
 
-      // __(key, value)
+      // __(locale, key, value)
       return util.format(text, value);
     }
 
-    // __(key, value1, ...)
-    const args = new Array(arguments.length);
+    // __(locale, key, value1, ...)
+    const args = new Array(arguments.length - 1);
     args[0] = text;
-    for(let i = 1; i < args.length; i++) {
-      args[i] = arguments[i];
+    for(let i = 2; i < arguments.length; i++) {
+      args[i - 1] = arguments[i];
     }
     return util.format.apply(util, args);
+  }
+
+  app[functionName] = gettext;
+
+  app.context[functionName] = function (key, value) {
+    if (arguments.length === 0) {
+      // __()
+      return '';
+    }
+
+    const locale = this.__getLocale();
+    if (arguments.length === 1) {
+      return gettext(locale, key);
+    }
+    if (arguments.length === 2) {
+      return gettext(locale, key, value);
+    }
+    const args = new Array(arguments.length + 1);
+    args[0] = locale;
+    for(let i = 0; i < arguments.length; i++) {
+      args[i + 1] = arguments[i];
+    }
+    return gettext.apply(this, args);
   };
 
   // 1. query: /?locale=en-US

test/index.test.js

@@ -5,10 +5,9 @@ const koa = require('koa');
 const request = require('supertest');
 const pedding = require('pedding');
 const mm = require('mm');
-const locales = require('../');
+const locales = require('..');
 
 describe('koa-locales.test.js', function () {
-
   afterEach(mm.restore);
 
   describe('default options', function () {
@@ -122,6 +121,15 @@ describe('koa-locales.test.js', function () {
       .expect(200, done);
     });
 
+    it('should gettext work on app.__(locale, key, value)', function (done) {
+      request(app.callback())
+      .get('/app_locale_zh')
+      .expect({
+        email: '邮箱1',
+      })
+      .expect(200, done);
+    });
+
     describe('query.locale', function () {
       it('should use query locale: zh-CN', function (done) {
         request(app.callback())
@@ -568,6 +576,13 @@ function createApp(options) {
   const fname = options && options.functionName || '__';
 
   app.use(function* () {
+    if (this.url === '/app_locale_zh') {
+      this.body = {
+        email: this.app[fname]('zh-cn', 'Email'),
+      };
+      return;
+    }
+
     if (this.path === '/origin') {
       assert(this.__getLocaleOrigin() === this.__getLocaleOrigin());
       this.body = {