Handle Models where Config.extra is not set

[joe0BAB]

Hi! First of all, many thanks for providing this useful tool!

I'm trying to get it running, but there are two issues I'm stuck with.

First is the same as #5

The second, when I pass the path of a python file directly I get:
`$pydantic2ts --module ./api/apps/folder/models.py --output test.ts --json2ts-cmd ./ui/node_modules/.bin/json2ts
2021-05-11 15:47:53,683 Finding pydantic models...
2021-05-11 15:47:53,688 Generating JSON schema from pydantic models...
Traceback (most recent call last):
File ".../venv/bin/pydantic2ts", line 8, in
sys.exit(main())
File ".../venv/lib/python3.8/site-packages/click/core.py", line 829, in call
return self.main(*args, **kwargs)
File ".../venv/lib/python3.8/site-packages/click/core.py", line 782, in main
rv = self.invoke(ctx)
File ".../venv/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
return ctx.invoke(self.callback, **ctx.params)
File ".../venv/lib/python3.8/site-packages/click/core.py", line 610, in invoke
return callback(*args, **kwargs)
File ".../venv/lib/python3.8/site-packages/pydantic2ts/cli/script.py", line 222, in main
return generate_typescript_defs(module, output, json2ts_cmd)
File ".../venv/lib/python3.8/site-packages/pydantic2ts/cli/script.py", line 187, in generate_typescript_defs
schema = generate_json_schema(models)
File ".../venv/lib/python3.8/site-packages/pydantic2ts/cli/script.py", line 144, in generate_json_schema
model_extras = [m.Config.extra for m in models]
File ".../venv/lib/python3.8/site-packages/pydantic2ts/cli/script.py", line 144, in
model_extras = [m.Config.extra for m in models]

AttributeError: type object 'Config' has no attribute 'extra'
`
If I understand the docs correctly, the Config.extra field is optional.
However, right now pydantic-to-typescript does not handle this case.