templateExtension configuration option.

As agreed, this changes won't break public API and by default (i.e. when ```templateExtension``` option is not specified in karma config) library will behave exactly as it used to.

Here is how I use it myself:

```
    ngJade2JsPreprocessor: {
      stripPrefix: '.*/source-path',
      templateExtension: 'jade',
      moduleName: 'templates'
    },
```

I used ```templateExtension``` instead of ```keepOriginalExtension``` (boolean), because it gives more flexibility to users.