Indicate proxy interfaces are Functional Interfaces (#16)

* Update RequestInterceptor.java

Add @FunctionalInterface in interceptor definition to give user indication of using lambdas in most IDEs

* Update ResponseInterceptor.java

Add @FunctionalInterface in interceptor definition to give user indication of using lambdas in most IDEs

src/main/java/website/magyar/mitm/proxy/RequestInterceptor.java

@@ -7,6 +7,7 @@
  *
  * @author Tamas Kohegyi
  */
+@FunctionalInterface
 public interface RequestInterceptor {
     void process(MitmJavaProxyHttpRequest request);
 }

src/main/java/website/magyar/mitm/proxy/ResponseInterceptor.java

@@ -7,6 +7,7 @@
  *
  * @author Tamas Kohegyi
  */
+@FunctionalInterface
 public interface ResponseInterceptor {
     void process(MitmJavaProxyHttpResponse response);
 }