ES6 let and const syntax

[adidahiya]

microsoft/TypeScript#904

[adidahiya]

In order to properly address #321, it seems like we need a BlockScopeAwareRuleWalker<T> and a new rule to lint duplicate let declarations in the same block scope. I don't see a clean way to integrate this into NoDuplicateVariableWalker while still supporting valid syntax like:

let a = 1;
if (true) {
    let a = 2;
}

... so I propose a new NoDuplicateLetWalker. Thoughts @gscshoyru, @ashwinr?

UPDATE: implemented here #374
UPDATE 2: see #375 instead; this has been integrated into the no-duplicate-variable rule