* Create a sources package to encode the convention followed in our templates to create a WebHost.
* Add an extension method to setup the content root relative to the solution folder.
* Add a factory method to create a WebHostBuilder based on the pattern followed in our template.
* Add IStartupConfigureServicesFilter to wrap ConfigureServices.
* Add IStartupConfigureContainerFilter<TContainerBuilder> to wrap
ConfigureContainer.
* Make StartupLoader build a thunk for configure services that
resolves all instances of IStartupConfigureServicesFilter and
IStartupConfigureContainerFilter<TContainerBuilder> and wraps
invocations to ConfigureServices and ConfigureContainer respectively.
* Refactor building the ConfigureServices callback into a private
builder class due to the increased complexity in the process.
Javier Calvarro Nelson