feat(preprocessors): allow multiple file patterns matching

allows to specify multiple file patterns in the preprocessors config so you can add exclusions rules. useful if you want for example coverage to match all src/**/*.js files except those ending with *.spec.js.

Should help for https://github.com/karma-runner/karma-coverage/issues/13

BREAKING CHANGE: the preprocessors config order has been reversed.

Before :

```js
preprocessors: {
  '**/*.coffee': ['coffee'],
  '**/*.js': ['coverage']
}
```

After :

```js
preprocessors: {
  'coffee': '**/*.coffee',
  'coverage': ['**/*.js', '!**/*.spec.js']
}
```