Add ignore comments in file (fixes #305)

I guess it would be useful to have possibility to suppress error messages for specific lines in file. Not it can be made as 
```js
alert('test'); // message here
/*eslint-ignore*/
alert('test'); // nothing here
/*eslint-end-ignore */
```
or for specific rules

```js
alert('test'); // message here
/*eslint-ignore no-alert, strict */
alert('test'); // nothing here
console.log('test') //message here
/*eslint-end-ignore */
```

I don't want to make special `/*eslint-end-ignore rule*/`, I guess it would be too complicated for user to track ignore then.

I'll be glad to have feedback, maybe comments should have better names or something.