Client Configuration (#219)

Added support for IOptions injection, allowing options to be read from IConfiguration
Small refactor on client options internals
Updated HttpClient to be static field to be

CryptoExchange.Net.UnitTests/TestImplementations/TestBaseClient.cs

@@ -20,7 +20,7 @@ namespace CryptoExchange.Net.UnitTests
 
         public TestBaseClient(): base(null, "Test")
         {
-            var options = TestClientOptions.Default.Copy();
+            var options = new TestClientOptions();
             Initialize(options);
             SubClient = AddApiClient(new TestSubClient(options, new RestApiOptions()));
         }