Suggest that the stylesheet be set if not provided

  * also stops application crash when stylesheet is not defined
Merge pull request #13 from squidpunch/stylesheet-guard

Suggest that the stylesheet be set if not provided
changed append to insert above ./app

should be right above app, so RMQ and ProMotion have done their work.
In order to make append syntax works on all type of ProMotion Screen, like TableScreen, it should be extend under ProMotion::ScreenModule not ProMotion::Screen
Merge pull request #15 from chunlea/extend_append_syntax_under_screen_module

Make append works on more screen type like PM::TableScreen,
Merge pull request #14 from GantMan/master

changed append to insert above ./app