Simplify MvcOptions

[rynowak]

From API review:

We want to move away from OptionDescriptor<T>. The desire here is to simplify the lifecycle for the various components that go into options - in general they should be instances/singletons.

There may be some cases where the non-singleton makes sense (IViewEngine for one), in that case evaluate the best pattern (factory, type activation, service).

We want to keep extension methods that we've built where the do something useful. In general the properties on options will be an IList<T> and will be easier to work with overall.