Add support for customizing the view renderers used

It'd be nice to have the ability to customize the ViewRenderers that are used in DropWizard. 

I created a [Handlebars ViewRenderer](https://github.com/tburch/dropwizard-extras/tree/feature/handlebars/dropwizard-extras-view/dropwizard-extras-view-handlebars) for Dropwizard, but I'm unable to pass in an instance of Handlebars in the ViewRenderer because of how the ViewMessageBodyWriter instantiates ViewRenderer instances. In the Mustache and FreeMarker cases, this is ok, but for Handlebars, using the default constructor is insufficient because you wouldn't be able to register any customer helpers or caching strategies with it.