Hosting renaming WebApplication to WebHost

[JunTaoLuo]

All classes prefixed with WebApplication have been renamed to WebHost. This includes:

• IWebApplicationBuilder => IWebHostBuilder
• WebApplicationBuilder => WebHostBuilder
• IWebApplication => IWebHost
• WebApplication => WebHost
• WebApplicationOptions => WebHostOptions
• WebApplicationDefaults => WebHostDefaults
• WebApplicationService => WebHostService
• WebApplicationConfiguration => WebHostConfiguration

Changes other than the direct renames include:

• WebApplicationBuilderExtensions has been split up into WebHostBuilderExtensions and WebHostExtensions to more accurately describe which class the extension methods are for.
• WebApplicationExtensions => WebHostWindowsServiceExtensions
• Added extensions .UseDefaultConfiguration(string[] args) on IWebHostBuilder. This call is equivalent to the old usage builder.UseConfiguration(WebApplicationConfiguration.GetDefault(args)). An overload that does not take the string[] args argument is also provided.

Please use aspnet/Hosting#567 for discussion.