Support mustache template caching

According to the JMustache docs, all the necessary classes are thread safe with the caveat that if you customize any classes they also have to be thread safe.

Currently instances of the MustacheView class can be cached, but not the result of compilation. Every time the view is rendered in renderMergedTemplateModel, the resource gets loaded, parsed and compiled into a Template before rendering. If there was an option to cache the compiled form, perhaps with a pluggable caching implementation, we wouldn't need to load and compile on every request. Admittedly I didn't do any performance testing, but it seems like it should be beneficial.

While I'm on the subject of the MustacheView class, it strikes me as odd that it uses a getApplicationContext().getResource(...) to get the resource instead of a MustacheResourceTemplateLoader instance. After all, when using partials, it will load the partial using the MustacheResourceTemplateLoader.

Start in MustacheView, especially renderMergedTemplateModel, to trace where the resource is loaded, parsed, and compiled on each render. Compare that path with MustacheResourceTemplateLoader and its partial loading behavior. Done means a defined option for caching compiled templates, potentially with a pluggable cache, and a resolved approach for resource loading.