Add headers arg to urljson

[CarloLepelaars]

Hi @jph00, I love the simplicity of urljson and use it often, but unfortunately it doesn't support headers.

Currently we can achieve the same behavior with urlread:

urlread("https://example.com", headers={"API_KEY": key}, return_json=True)

but with the proposed change it becomes slightly more elegant

urljson("https://example.com", headers={"API_KEY": key})

[jph00]

Thank you! :)