Add support for custom anchor element attributes to fmt_url()

[elipousson]

Prework

• Read and abide by gt's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

I recently realized that the fmt_url() function does not allow custom attribute values and has the target attribute hard-coded as "_blank". Adding support for custom attributes to the fmt_url() function would add greater flexibility how the links are created. Support for other custom attributes including hreflang, ping, referrerpolicy, rel, and type could be added at the same time.

I used the documentation on the supported attributes for anchor elements ("") from the mdn developer docs as a reference in setting up the validation: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a#attributes

[elipousson]

I think I've got a decent implementation for this feature ready to go on my fork, @rich-iannone.

If it looks good for a pull request, just let me know and I can add some tests as well. If you'd recommend a different approach for implementation, feedback would be welcome.