Support trailing commas in function call/declaration

With love to @ptarjan

README.md

@@ -30,6 +30,7 @@ Phack extends [PHP-Parser](https://www.github.com/nikic/PHP-Parser) by amending
 | [Shapes](https://docs.hhvm.com/hack/shapes) | TBD | |
 | [Collections](https://docs.hhvm.com/hack/collections) | TBD | Static initialization is... complicated... |
 | [Async](https://docs.hhvm.com/hack/async) | Unlikely | PHP's Engine just doesn't support this concept well |
+| Trailing Commas | Alpha | Allow trailing comma in arg/param lists |
 
 ## How do I use it?
 

grammar/hack-extras.y

@@ -70,6 +70,7 @@ hack_parameter_type_list:
 
 hack_parameter_list:
 	  hack_non_empty_parameter_list { $$ = $1; }
+	| hack_non_empty_parameter_list ',' { $$ = $1; }
 	| /* empty */ { $$ = array(); }
 ;
 
@@ -92,6 +93,14 @@ hack_optional_visibility_modifier:
     | T_PRIVATE { $$ = Stmt\Class_::MODIFIER_PRIVATE; }
 ;
 
+parameter_list:
+	  non_empty_parameter_list ',' { $$ = $1; }
+;
+
+argument_list:
+	  '(' non_empty_argument_list ',' ')' { $$ = $2; }
+;
+
 type:
 	  '?' type { $$ = PhackNode\SoftNullableType[$2, false, true]; }
 	| '@' type { $$ = PhackNode\SoftNullableType[$2, true, false]; }