add undefinedToVoid optional transformer

lib/6to5/transformation/transform.js

@@ -81,6 +81,7 @@ _.each({
   _declarations:             require("./transformers/_declarations"),
 
   coreAliasing:              require("./transformers/optional-core-aliasing"),
+  undefinedToVoid:           require("./transformers/optional-undefined-to-void"),
 
   // wrap up
   _aliasFunctions:           require("./transformers/_alias-functions"),

test/fixtures/transformation/optional-undefined-to-void/basic/actual.js

@@ -0,0 +1,2 @@
+var foo;
+foo === undefined;

test/fixtures/transformation/optional-undefined-to-void/basic/expected.js

@@ -0,0 +1,4 @@
+"use strict";
+
+var foo;
+foo === void 0;

test/fixtures/transformation/optional-undefined-to-void/member-expression/actual.js

@@ -0,0 +1,2 @@
+var foo;
+foo === undefined.foo;

test/fixtures/transformation/optional-undefined-to-void/member-expression/expected.js

@@ -0,0 +1,4 @@
+"use strict";
+
+var foo;
+foo === (void 0).foo;

test/fixtures/transformation/optional-undefined-to-void/options.json

@@ -0,0 +1,3 @@
+{
+  "optional": ["undefinedToVoid"]
+}