give the option to change default branch for News.md (and others)

[AliSajid]

Hi,

I have just started using codemetar and I find it an excellent resource. It is a pre-commit hook so I don't worry about it. However, a major issue for me was when I ran it, it generated releasenotes URL as having master in the URL. With a bit of digging I found out that that was hard coded but could be overwritten using an options setting.

I wonder if it could be made an actual option in write_codemeta or it could be more prominently noted in the docs. Happy to contribute whichever way is decided. Thanks.

[maelle]

@cboettig what do you think?

Thanks @AliSajid for catching this 🙏