[ES7] Trailing comma in function parameter list

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

src/expression.js

@@ -224,7 +224,7 @@ pp.parseSubscripts = function(base, start, noCalls) {
   } else if (!noCalls && this.eat(tt.parenL)) {
     let node = this.startNodeAt(start)
     node.callee = base
-    node.arguments = this.parseExprList(tt.parenR, false)
+    node.arguments = this.parseExprList(tt.parenR, this.options.features["es7.trailingFunctionCommas"])
     return this.parseSubscripts(this.finishNode(node, "CallExpression"), start, noCalls)
   } else if (this.type === tt.backQuote) {
     let node = this.startNodeAt(start)

src/statement.js

@@ -458,7 +458,7 @@ pp.parseFunction = function(node, isStatement, allowExpressionBody, isAsync) {
 
 pp.parseFunctionParams = function(node) {
   this.expect(tt.parenL)
-  node.params = this.parseBindingList(tt.parenR, false, false)
+  node.params = this.parseBindingList(tt.parenR, false, this.options.features["es7.trailingFunctionCommas"])
 }
 
 // Parse a class declaration or literal (depending on the