Optional kv_file argument to address Issue #497 Remove magic for kv inclusion

This pull request contains a tiny example implementation of an optional kv_file argument to be passed into App.run.

i.e. PongApp.run(kv_file = 'my_oddly_name_file.kv')

Issue #497 'Remove magic for kv inclusion' is currently stalled on backwards-compatibility concerns. This patch contains a possible work-around for those concerns - by leaving the magic intact, but allowing it to be replaced with an explicitly specified file when needed.