added 'let' keyword to JavascriptLexer
added 'let' keyword to JavascriptLexer

pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments,pathawks/pygments