TypeError when readme option is not set

[thomasgilgenast]

As mentioned in #5, if the readme option in the [target] section of the config file is not set, treeshaker crashes with:

Traceback (most recent call last):
  File ".../venv/bin/treeshaker", line 8, in <module>
    sys.exit(main())
  File ".../venv/lib/python3.8/site-packages/treeshaker/treeshaker.py", line 293, in main
    fire.Fire(run_from_config)
  File ".../venv/lib/python3.8/site-packages/fire/core.py", line 141, in Fire
    component_trace = _Fire(component, args, parsed_flag_args, context, name)
  File ".../venv/lib/python3.8/site-packages/fire/core.py", line 466, in _Fire
    component, remaining_args = _CallAndUpdateTrace(
  File ".../venv/lib/python3.8/site-packages/fire/core.py", line 681, in _CallAndUpdateTrace
    component = fn(*varargs, **kwargs)
  File ".../venv/lib/python3.8/site-packages/treeshaker/treeshaker.py", line 284, in run_from_config
    readme=os.path.join(config_path, section['readme']),
  File ".../lib/python3.8/posixpath.py", line 90, in join
    genericpath._check_arg_types('join', a, *p)
  File ".../lib/python3.8/genericpath.py", line 152, in _check_arg_types
    raise TypeError(f'{funcname}() argument must be str, bytes, or '
TypeError: join() argument must be str, bytes, or os.PathLike object, not 'NoneType'

This happens because there is no logic in place on this line to check if section['readme'] is None:

treeshaker/treeshaker/treeshaker.py

Line 284 in a116fc9

readme=os.path.join(config_path, section['readme']),

But in process_module() itself we do have logic to check if readme is None:

treeshaker/treeshaker/treeshaker.py

Lines 207 to 220 in a116fc9

	# load readme content
	readme_content = None
	if readme and os.path.exists(readme):
	with open(readme, 'r') as handle:
	readme_content = handle.read()
	# write README
	if readme_content or fire_components or functions:
	new_name = old_name_to_new_name[target_module_name]
	with open(os.path.join(dest_dir, 'README.md'), 'w') as handle:
	handle.write('%s\n' % dest_dir)
	handle.write(('=' * len(dest_dir)) + '\n\n')
	if readme_content:
	handle.write(readme_content + '\n')