'Add a new hook of specified type.
:param cls: class that registers hooks
:param hook_type: hook type, e.g., \'__pre_parse_args__\'
:param hook_func: hook function'

@classmethod
def add_hook(cls, hook_type, hook_func):

    if (hook_type not in cls._hooks_map):
        cls._hooks_map[hook_type] = []
    cls._hooks_map[hook_type].append(hook_func)
