Make the HostContext available to extension methods on IWebHostBuilder

[davidfowl]

Today it's impossible to get at the HostingContext as an extension method on IWebHostBuilder, this means you can't flow the IHostingEnvironment in your extension methods that hang off the builder.

/cc @JunTaoLuo @glennc

[glennc]

I was thinking of a couple of things around this:

1. promote WebHostBuilderContext to abstractions and have it as a property of the builder
2. provide a UseServices extension that has both WebHostBuilderContext and ServiceCollection. So that at least extensions that are just adding services can do it trivially.

What do you think of those?

[davidfowl]

I don't think we need 2 types (maybe that was a typo?). UseServices shouldn't be required to get access to the host context, and it should be called ConfigureServices right?

[JunTaoLuo]

Thanks @glennc for the PR #1020