Updated CSHARP_KEYWORDS

Updated `CSHARP_KEYWORDS` section with the new keywords introduced in C# 6.
Based on the official list of [C# keywords](https://msdn.microsoft.com/en-us/library/x53a06bb.aspx).

Author: weitzhandler

src/prettify.js

@@ -108,11 +108,12 @@ var prettyPrint;
       "instanceof,interface,null,native,package,strictfp,super,synchronized," +
       "throws,transient"];
   var CSHARP_KEYWORDS = [COMMON_KEYWORDS,
-      "abstract,as,base,bool,by,byte,checked,decimal,delegate,descending," +
-      "dynamic,event,finally,fixed,foreach,from,group,implicit,in,interface," +
-      "internal,into,is,let,lock,null,object,out,override,orderby,params," +
-      "partial,readonly,ref,sbyte,sealed,stackalloc,string,select,uint,ulong," +
-      "unchecked,unsafe,ushort,var,virtual,where"];
+      "abstract,add,alias,as,ascending,async,await,base,bool,by,byte,checked," +
+      "decimal,delegate,descending,dynamic,event,finally,fixed,foreach,from," +
+      "get,global,group,implicit,in,interface,internal,into,is,join,let,lock," +
+      "null,object,out,override,orderby,params,partial,readonly,ref,remove," +
+      "sbyte,sealed,select,set,stackalloc,string,uint,ulong,unchecked,unsafe," +
+      "ushort,value,var,virtual,where,yield"];
   var COFFEE_KEYWORDS = "all,and,by,catch,class,else,extends,false,finally," +
       "for,if,in,is,isnt,loop,new,no,not,null,of,off,on,or,return,super,then," +
       "throw,true,try,unless,until,when,while,yes";