xmp: Added support for additional metadata

[LNA-DEV]

This adds support for a couple of additional metadata properties in XMP. All of these are children of the RdfDescription

XMP subject not supported #40

[bep]

I'm going to close this for now, as I think this goes beyond the scope of this project (at least it goes beyond the scope of mt free time).

See https://pkg.go.dev/github.com/bep/imagemeta#Options and HandleXMP for how to handle this on the caller side,.

[LNA-DEV]

Okey I can absolutely understand that. But this implies that some tags won't be supported downstream in Hugo am I right?

[bep]

I'll reopen this and have a look at it later (but no promise) I was a little bit grumpy last night ...

[LNA-DEV]

Thanks for reconsidering^^ I appreciate it a lot!

To make my usecase clear: I want to use the metadata saved by my photoeditor (Darktable) to build the gallery section of my webpage. (So I can set an alt text and "hashtags" based on the Darktable metadata.) This PR together with gohugoio/hugo#13539 on the Hugo side would enable that use. (I am currently testing it with a custom build of this two PRs. Looks good so far.)

After all I can totally understand if it is too much of an edge case but I think this could be useful for image centered pages.

[bep]

I couldn't push to this PR, so I replaced it with #43