[ES7] Trailing comma in function parameter list

Currenly a stage 1 proposal.
See https://github.com/jeffmo/es-trailing-function-commas.

test/core/fixtures/transformation/es7.trailing-function-commas/call/actual.js

@@ -0,0 +1,4 @@
+Math.max(1,
+         2,
+         3,
+);

test/core/fixtures/transformation/es7.trailing-function-commas/call/expected.js

@@ -0,0 +1,3 @@
+"use strict";
+
+Math.max(1, 2, 3);

test/core/fixtures/transformation/es7.trailing-function-commas/declaration/actual.js

@@ -0,0 +1,8 @@
+function thisIsAFunctionWithAVeryLongNameAndWayTooManyParameters(
+  thisIsTheFirstParameter, andThisOneIsRelatedToIt,
+  butNotThisOne,
+  andNeitherThis,
+  inFactThereArentThatManyParameters,
+) {
+  throw null;
+}

test/core/fixtures/transformation/es7.trailing-function-commas/declaration/expected.js

@@ -0,0 +1,5 @@
+"use strict";
+
+function thisIsAFunctionWithAVeryLongNameAndWayTooManyParameters(thisIsTheFirstParameter, andThisOneIsRelatedToIt, butNotThisOne, andNeitherThis, inFactThereArentThatManyParameters) {
+  throw null;
+}

test/core/fixtures/transformation/es7.trailing-function-commas/options.json

@@ -0,0 +1,3 @@
+{
+  "optional": ["es7.trailingFunctionCommas"]
+}