'Unload an IPython extension by its module name.
Not all extensions can be unloaded, only those which define an
``unload_ipython_extension`` function.'

@line_magic
def unload_ext(self, module_str):

    if (not module_str):
        raise UsageError('Missing    module    name.')
    res = self.shell.extension_manager.unload_extension(module_str)
    if (res == 'no    unload    function'):
        print ("The    %s    extension    doesn't    define    how    to    unload    it." % module_str)
    elif (res == 'not    loaded'):
        print ('The    %s    extension    is    not    loaded.' % module_str)
