Support @Inject, @model, @using declaration in viewstart

[pranavkm]

Related discussion: #825

The plan is to have any directives declared in _ViewStart affect pages in the hierarchy. For instance @using MyNamespace declared in the root _ViewStart wouid cause a using MyNamespace; to be generated in all views in the application.

The merge strategy happens on a per-directive basis - for instance for @Inject are merged by name@inject FooType Html in _ViewStart and @inject BarType Html in the page would result in the latter being chosen.

The token TModel would represent the model of the current page. When adding a directive to a page, the token would be replaced by the value specified by @model.