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) and [Markdown ViewRenderer](https://github.com/tburch/dropwizard-extras/tree/0419821e64ce01501d67cb07c9ba9c45909dc7ec/dropwizard-extras-view/dropwizard-extras-view-markdown) for Dropwizard, but I'm unable to pass in an instance of Handlebars/Markdown4jProcessor in the ViewRenderer because of how the ViewMessageBodyWriter instantiates ViewRenderer instances. In the Mustache and FreeMarker cases, this is ok, but for Handlebars/Markdown, using the default constructor is insufficient because you wouldn't be able to customize the rendering engine being used.