Add support for metalsmith builder hooks

Close vuejs#427

Author: Toilal

README.md

@@ -77,6 +77,8 @@ vue init ~/fs/path/to-custom-template my-project
   - `prompts`: used to collect user options data;
 
   - `filters`: used to conditional filter files to render.
+  
+  - `metalsmith`: used to add custom metalsmith plugins in the chain.
 
   - `completeMessage`: the message to be displayed to the user when the template has been generated. You can include custom instruction here.
 
@@ -179,6 +181,36 @@ The `skipInterpolation` field in the metadata file should be a [minimatch glob p
 }
 ```
 
+#### Metalsmith
+
+`vue-cli` uses [metalsmith](https://github.com/segmentio/metalsmith) to generate the project.
+
+You may customize the metalsmith builder created by vue-cli to register custom plugins.
+
+```javascript
+{
+  "metalsmith": function(metalsmith, opts, helpers) {
+    function customMetalsmithPlugin(files, metalsmith, done) {
+      // Implement something really custom here.
+      done(null, files);
+    }
+    
+    metalsmith.use(customMetalsmithPlugin);
+  }
+}
+```
+
+If you need your to hook metalsmith before questions are asked, you may use an object with `before`/`after` keys
+
+```javascript
+{
+  "metalsmith": {
+    before: function(metalsmith, opts, helpers) {},
+    after: function(metalsmith, opts, helpers) {}
+  }
+}
+```
+
 #### Additional data available in meta.{js,json}
 
 - `destDirName` - destination directory name

lib/generate.js

@@ -43,11 +43,24 @@ module.exports = function generate (name, src, dest, done) {
   opts.helpers && Object.keys(opts.helpers).map(function (key) {
     Handlebars.registerHelper(key, opts.helpers[key])
   })
-  metalsmith
-    .use(askQuestions(opts.prompts))
+
+  var helpers = {chalk, logger}
+
+  if (opts.metalsmith && typeof opts.metalsmith.before === 'function') {
+    opts.metalsmith.before(metalsmith, opts, helpers)
+  }
+
+  metalsmith.use(askQuestions(opts.prompts))
     .use(filterFiles(opts.filters))
     .use(renderTemplateFiles(opts.skipInterpolation))
-    .clean(false)
+
+  if (typeof opts.metalsmith === 'function') {
+    opts.metalsmith(metalsmith, opts, helpers)
+  } else if (opts.metalsmith && typeof opts.metalsmith.after === 'function') {
+    opts.metalsmith.after(metalsmith, opts, helpers)
+  }
+
+  metalsmith.clean(false)
     .source('.') // start from template root instead of `./src` which is Metalsmith's default for `source`
     .destination(dest)
     .build(function (err, files) {