Add extension generator command.

stubs/extension/js/.gitignore

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+bower_components
+node_modules
+dist

stubs/extension/js/Gulpfile.js

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+var gulp = require('flarum-gulp');
+
+gulp({
+  modulePrefix: '{{name}}'
+});

stubs/extension/js/bootstrap.js

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+import { extend, override } from 'flarum/extension-utils';
+
+app.initializers.add('{{name}}', function() {
+
+  // @todo
+
+});

stubs/extension/js/package.json

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+{
+  "private": true,
+  "devDependencies": {
+    "gulp": "^3.8.11",
+    "flarum-gulp": "git+https://github.com/flarum/gulp.git"
+  }
+}