Use interpret to support multiple format module loading in karma.conf.js

### Expected behavior

karma auto loads karma.conf.babel.js or karma.conf.ts etc.
### Actual behavior

you have to create a karma.conf.js that dispatches to karma.conf.babel.js or karam.conf.ts.
### Enviroment Details
- Karma version (output of `karma --version`):

```
zsh: command not found: karma
```
- Relevant part of your `karma.config.js` file

``` js
export default function configure(config) {
...
}
```