Nested routes

Fixes #262

test/app/routes/_layout.html

@@ -0,0 +1,15 @@
+{#if preloading}
+	<progress class='preloading-progress' value=0.5/>
+{/if}
+
+<svelte:component this={child.component} {rootPreloadFunctionRan} {...child.props}/>
+
+<script>
+	export default {
+		preload() {
+			return {
+				rootPreloadFunctionRan: true
+			};
+		}
+	};
+</script>