Implement withEnvironment method. (#63)

* Implement withEnvironment method.

* Add withEnvironment test

* Remove unnecessary default callback from withEnvironment

lib/run-context.js

@@ -80,7 +80,10 @@ RunContext.prototype._run = function() {
 
   this.ran = true;
   var namespace;
-  this.env = yeoman.createEnv([], {}, new TestAdapter());
+  this.env = (this.envCB || (env => env)).call(
+    this,
+    yeoman.createEnv([], {}, new TestAdapter())
+  );
 
   helpers.registerDependencies(this.env, this.dependencies);
 
@@ -217,6 +220,20 @@ RunContext.prototype.inTmpDir = function(cb) {
   return this.inDir(tmpdir, cb);
 };
 
+/**
+ * Create an environment
+ *
+ * This method is called automatically when creating a RunContext. Only use it if you need
+ * to use the callback.
+ *
+ * @param {Function} [cb] - callback who'll receive the folder path as argument
+ * @return {this} run context instance
+ */
+RunContext.prototype.withEnvironment = function(cb) {
+  this.envCB = cb;
+  return this;
+};
+
 /**
  * Clean the directory used for tests inside inDir/inTmpDir
  */

test/run-context.js

@@ -628,6 +628,23 @@ describe('RunContext', function() {
     });
   });
 
+  describe('#withEnvironment()', function() {
+    it('register paths', function(done) {
+      this.ctx
+        .withEnvironment(env => {
+          env.register(require.resolve('./fixtures/generator-simple/app'));
+          return env;
+        })
+        .on(
+          'ready',
+          function() {
+            assert(this.ctx.env.get('simple:app'));
+            done();
+          }.bind(this)
+        );
+    });
+  });
+
   describe('#withLocalConfig()', function() {
     it('provides config to the generator', function(done) {
       this.ctx