Added initial_url to the router component (#510)

* added active_url

* renamed active_url to initial_url

* updated docs

packages/router/src/cfg.rs

@@ -2,4 +2,5 @@
 pub struct RouterCfg {
     pub base_url: Option<String>,
     pub active_class: Option<String>,
+    pub initial_url: Option<String>,
 }