Add 6to5 colmun

For #281 .

I checked [this](https://github.com/e-jigsaw/6to5-es6-compat-table) inspired by [teppeis/closure-compiler-es6-compat-table](https://github.com/teppeis/closure-compiler-es6-compat-table).

But, some tests are maybe incorrect. For example `let`. Compiled code from `let` test raised error

```
[ReferenceError: __let_script_executed is not defined]
```

compiled code is

```javascript
// let
"use strict";
module.exports = function() {

            try {
              return (function () { "use strict"; __let_script_executed = true; var _foobarbaz2 = 123; return _foobarbaz2 == 123; }());
            } catch (e) {
              return false;
            }
          
};
```