Added group attribute to stop router from merging exception-handlers (issue: #573)

- Added new mergeExceptionHandlers attribute to stop router from merging exception-handlers.
- RouteGroup: Added setMergeExceptionHandlers and getMergeExceptionHandlers methods.
- IRouteGroup: Added setMergeExceptionHandlers and getMergeExceptionHandlers method.
- Updated documentation to reflect changes.
- Added unit-tests.

src/Pecee/SimpleRouter/Route/IGroupRoute.php

@@ -31,6 +31,21 @@ interface IGroupRoute extends IRoute
      */
     public function setExceptionHandlers(array $handlers): self;
 
+    /**
+     * Returns true if group should overwrite existing exception-handlers.
+     *
+     * @return bool
+     */
+    public function getMergeExceptionHandlers(): bool;
+
+    /**
+     * When enabled group will overwrite any existing exception-handlers.
+     *
+     * @param bool $merge
+     * @return static
+     */
+    public function setMergeExceptionHandlers(bool $merge): self;
+
     /**
      * Get exception-handlers for group
      *