[core] add openfl preloader param

template/project.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <project>
     <meta title="<%=meta.title%>" package="<%=meta.pack%>" version="<%=meta.version%>" company="<%=meta.company%>"/>
-    <app main="<%=main%>" path="<%=buildDir%>" file="<%=meta.filename%>"/>
+    <app main="<%=main%>" path="<%=buildDir%>" file="<%=meta.filename%>" preloader="<%=preloader%>"/>
     <icon path="<%=icon%>"/>
     <% sources.forEach(function(item) { %>
     <source path="<%=item%>"/><% }); %>