add adding metadata capability

[Myfanwy]

In particular, image titles (for inclusion within a photo database that has standardized filenames, for example) and tags (searchable subject tags, i.e. "fish" or "field equipment"

[jhollist]

I don't know this for certain, but I wonder if a lot of this is available in https://cran.r-project.org/package=magick. I haven't had a chance to look...

[Ironholds]

I see a thing to read image metadata but nothing to write it.