let not detected

[Philipp Söhnlein]

let foo = 'a'; isn't considered problematic.
That's probably because it's ES6, so the issue title should maybe be I can haz ES6 support?.

[Thijs Busser]

Thanks for the report, I'll be sure to add detection for this!

[Thijs Busser]

Detection for let has been added along with detection for const.