Execution after validation failed. 
Hi,
I'm writing complexity validator.
But I found that still processing query after return 400.
And the point is in koa-graphgl/dist/index.js line 132 ~ 138

```javascript
        // Validate AST, reporting any errors.
        var validationErrors = (0, _graphql.validate)(schema, documentAST, validationRules);
        if (validationErrors.length > 0) {
          // Return 400: Bad Request if any validation errors exist.
          response.status = 400;
          resolve({ errors: validationErrors });
        }
```
And it can simple change by add return after calling resolve.

In express-graphql implementation it will not continue execution after validation failed.

Maybe we can add a option to determine whether it should be execute after validation failed or not.
It would be great.