Define SPI for hooking into the view resolution

This issue is about starting some first discussions about defining a SPI for hooking into the view resolution. This idea was brought up by @jslsolucoes as part of #126.

The idea is to have something like:

```java
public interface ViewResolver {
Viewable resolve(ViewResolverContext ctx);
}

public interface ViewResolverContext {
Object getControllerResult();
ResourceInfo getResourceInfo();
// ....
}
```

This would for example allow to automatically derive the view name from the controller class/method name. Of course in this case the default implementation should be moved into an implementation of that interface. This will also require to add some kind of ordering or a chain like processing so that custom implementation can access the result of the default algorithm.

Feel free to share your thoughts.