Add variables def to simpleapp template

priv/templates/simpleapp.template

@@ -1,3 +1,4 @@
+{variables, [{appid, "myapp"}]}.
 {file, "simpleapp.app", "ebin/{{appid}}.app"}.
 {file, "simpleapp_app.erl", "src/{{appid}}_app.erl"}.
 {file, "simpleapp_sup.erl", "src/{{appid}}_sup.erl"}.