Feature/use any id server for admin area (#232)

* initial commits around using any id servers

* add your own id server for admin area

* lots of refactoring, now instead of injecting IWebHostBuilder we just set the Ocelot base url as a configuration extension method..this means people can pass it in on the command line aswell as hardcode which is OK I guess, also can now use your own IdentityServer to authenticate admin area

* updated docs for #231

* some tests that hopefully bump up coverage

test/Ocelot.UnitTests/DependencyInjection/ConfigurationBuilderExtensionsTests.cs

@@ -0,0 +1,41 @@
+using Microsoft.Extensions.Configuration;
+using Ocelot.DependencyInjection;
+using Shouldly;
+using TestStack.BDDfy;
+using Xunit;
+
+namespace Ocelot.UnitTests.DependencyInjection
+{
+    public class ConfigurationBuilderExtensionsTests
+    {
+        private IConfigurationRoot _configuration;
+        private string _result;
+
+        [Fact]
+        public void should_add_base_url_to_config()
+        {
+            this.Given(x => GivenTheBaseUrl("test"))
+                .When(x => WhenIGet("BaseUrl"))
+                .Then(x => ThenTheResultIs("test"))
+                .BDDfy();
+        }
+
+        private void GivenTheBaseUrl(string baseUrl)
+        {
+            var builder = new ConfigurationBuilder()
+                .AddOcelotBaseUrl(baseUrl);
+
+            _configuration = builder.Build();
+        }
+
+        private void WhenIGet(string key)
+        {
+            _result = _configuration.GetValue("BaseUrl", "");
+        }
+
+        private void ThenTheResultIs(string expected)
+        {
+            _result.ShouldBe(expected);
+        }
+    }
+}