Directory improvements

Your implementation of load_dirs() using os.walk() is not guaranteed to load files in a consistent order. os.walk() returns files in the order stored on the file system - this can vary from machine to machine. This patch replaces os.walk() with code that loads files in alphabetical order. 

Also expose filename_as_namespace as a flag on ConfigManager (defaults to True).

Together, these changes allow pymlconf to be used to implement conf.d style config (where config files are prefixed with a 2-digit number for ordering).