include kwarg which accepts a string to separate -H k: v pairs

[gaucheph]

main use case for this is for requests which have very long headers or a lot of them such that the curlified request, in its one-liner form, can be difficult to parse

[ofw]

Thanks! I think a better API would be to introduce a flag pretty=True|False to pretty print curl command when each part is on separate line

[ofw]

Hi! If you think that this change is still relevant please rebase it onto the main branch.