Expand Microsoft.Extensions.Hosting support to allow work to be done prior to Run*Async()

[hellfirehd]

Is your feature request related to a problem? Please describe.
I often have startup tasks that must be performed prior to running the console. I usually hang these off IHost so that they can create IServiceScope and benefit from the rest of the DI configuration.

var host = CreateHostBuilder(args).Build();
await host.StartServiceBusAsync("QueueName");
await host.RunAsync().ConfigureAwait(false);

However, you can't call .Buid() twice so that doesn't work with RunCommandLineApplicationAsync<T>().

Describe the solution you'd like
I want to be able to do this:

class Program
{
    static async Task<Int32> Main(String[] args)
    {
        var host = CreateHostBuilder(args).Build();

        await host.StartServiceBus("QueueName");

        return await host.RunCommandLineApplicationAsync<Program>();
    }

    public static IHostBuilder CreateHostBuilder(String[] args) =>
        Host.CreateDefaultBuilder(args)
            .UseCommandLineApplication<Program>(args)
            .ConfigureServices((hostContext, services) =>
            {
                /* SNIP */
            });
}

Describe alternatives you've considered
I looked at #134 and #185. They both wanted the same thing I do but it was early days and it appears they kind of fizzled.

I have a pull request ready to go that implements the above. I'd love for you to take a look at it!

[natemcmaster]

The proposal seems fine to me. Let's continue to iterate on the implementation together as proposed in #409

[cbcrouse]

Hey @natemcmaster and @hellfirehd - I was able to address the issues of code duplication and missing tests. I've created another PR.

@hellfirehd your commits are still included in this, I just forked and cherry-picked.

[natemcmaster]

Resolved in #409, #415, and #416