allow bake tasks to provide theme as cli option

This PR enables developers to provide what CLI theme to use directly from CLI

```
Console/cake bake test model SampleModel -q --theme non_default_theme
```

This is very practical for automating generation of tests from CLI, as you otherwise can't skip the theme question if you have multiple themes in your app.

Considering that the TemplateTask actually checks for this param to be present, it seems to be an unintended omission that you can't pass it in to the bake classes. 

https://github.com/cakephp/cakephp/blob/2.4/lib/Cake/Console/Command/Task/TemplateTask.php#L173