add content negotiation feature, add context.ReadYAML and fix https://github.com/kataras/neffos/issues/1#issuecomment-515698536

Former-commit-id: 9753e3e45c7c24788b97814d3ecfb4b03f5ff414

_examples/websocket/basic/browser/index.html

@@ -53,6 +53,8 @@
         };
     }
 
+    const username = window.prompt("Your username?");
+
     async function runExample() {
         // You can omit the "default" and simply define only Events, the namespace will be an empty string"",
         // however if you decide to make any changes on this example make sure the changes are reflecting inside the ../server.go file as well.
@@ -70,6 +72,10 @@
                         addMessage(msg.Body);
                     }
                 }
+            },{
+                headers: {
+                    "X-Username": username,
+                }
             });
 
             // You can either wait to conenct or just conn.connect("connect")