Added support for multiple reporters to the `makeReport` task.

Hello!

I've added another option to the `makeReport` task: `reporters`. It takes the following form:
```
options: {
  reporters: {
    '<reporter-type>': <reporter-options>
  }
}
```
where:
  - `<reporter-type>` is a reporter type supported by Istanbul (lcov, text, text-summary...)
  - `<reporter-options>` is an options object for the specified reporter type or a truthy value if that reporter does not have any options or a falsy value if that reporter should be skipped.

If the `options.reporters` option is not specified, it will be created from `options` for backwards compatibility:
```js
reporters[options.type] = options;
```

For example, to recreate a result of `istanbul cover --report lcov --print both ...` one can configure the gruntfile like this:
```js
    makeReport: {
      src: lcovReportDir + '/*.json',
      options : {
        reporters: {
          'lcov': {dir: lcovReportDir},
          'text': true,
          'text-summary': true
        }
      }
    }
```

In action: https://travis-ci.org/morkai/h5.step/builds/4705195 (scroll down to **Running "makeReport" task**)