Add ConfigAwarePlaceholders class (#997)

* Add ConfigAwarePlaceholders class

This allows placeholder values to be sourced from IConfiguration in
addition to set values.

* Rework how IPlaceholders is decorated in container

test/Ocelot.UnitTests/appsettings.json

@@ -20,5 +20,10 @@
       "port": 5000,
       "hostName": "localhost"
     }
+  },
+  "BaseUrl": "http://foo-bar.co.uk",
+  "TestConfig": "foo",
+  "TestConfigNested":{
+    "Child": "foo"
   }
 }