feat: Allow multiple report types to be used

I wanted a way to run multiple reports at the same time.  Specifically I wanted to see a text summary in the console but still have a way to get to the full html report to see line level details. 

This change refactors the way the coverageReporter configuration works by adding a new 'reporters' property.  This property is a list of configurations for any reporters that should be used.  This allows the user to ask for multiple output types at the same time.

For example:

```javascript
coverageReporter: {
    dir : 'coverage/',
    reporters: [{
      type: 'html'
    }, {
      type: 'text'
    }, {
      type: 'cobertura'
    }]
}
```

This configuration outputs reports in html, text, and cobertura from the same run of karma.

Note: the code is designed to be backwards compatible with the older method of specifying the options.