Fix: Install required eslint plugin for "standard" guide (fixes #6656) (#6657)

`eslint-config-standard` peer depends on `eslint-plugin-promise`, so
let's install that for users.

Author: feross

lib/config/config-initializer.js

@@ -278,7 +278,7 @@ function getConfigForStyleGuide(guide) {
     var guides = {
         google: {extends: "google"},
         airbnb: {extends: "airbnb", plugins: ["react"]},
-        standard: {extends: "standard", plugins: ["standard"]}
+        standard: {extends: "standard", plugins: ["standard", "promise"]}
     };
 
     if (!guides[guide]) {

tests/lib/config/config-initializer.js

@@ -195,7 +195,7 @@ describe("configInitializer", function() {
             it("should support the standard style guide", function() {
                 var config = init.getConfigForStyleGuide("standard");
 
-                assert.deepEqual(config, {extends: "standard", installedESLint: true, plugins: ["standard"]});
+                assert.deepEqual(config, {extends: "standard", installedESLint: true, plugins: ["standard", "promise"]});
             });
 
             it("should throw when encountering an unsupported style guide", function() {