Support for LetStatement

It seems like this is again Spidermonkey-only feature, but there's also LetStatement type which can be generated by Spidermonkey engine. Esprima produces [more expected result](http://esprima.org/demo/parse.html?code=for%20(let%20i%3D0%3B%20i%3C3%3B%20i%2B%2B)%20%7B%7D) and this is the output by Spidermonkey's Reflect:

```js -e 'print(JSON.stringify(Reflect.parse("for (let i=0; i<3; i++) {}", {loc: false}), null, "  "))'```
```json
{
  "loc": null,
  "type": "Program",
  "body": [
    {
      "loc": null,
      "type": "LetStatement",
      "head": [
        {
          "loc": null,
          "type": "VariableDeclarator",
          "id": {
            "loc": null,
            "type": "Identifier",
            "name": "i"
          },
          "init": {
            "loc": null,
            "type": "Literal",
            "value": 0
          }
        }
      ],
      "body": {
        "loc": null,
        "type": "ForStatement",
        "init": null,
        "test": {
          "loc": null,
          "type": "BinaryExpression",
          "operator": "<",
          "left": {
            "loc": null,
            "type": "Identifier",
            "name": "i"
          },
          "right": {
            "loc": null,
            "type": "Literal",
            "value": 3
          }
        },
        "update": {
          "loc": null,
          "type": "UpdateExpression",
          "operator": "++",
          "argument": {
            "loc": null,
            "type": "Identifier",
            "name": "i"
          },
          "prefix": false
        },
        "body": {
          "loc": null,
          "type": "BlockStatement",
          "body": []
        }
      }
    }
  ]
}
```

[LetStatement](https://developer.mozilla.org/en-US/docs/SpiderMonkey/Parser_API#Statements) is similar to VariableDeclaration, so I combined them.