Allow `citar-citeproc-format-reference` to take an optional `style` arg

[bdarcus]

We could also include a couple/few minimal CSL styles for this, a al here.

See https://github.com/pprevos/citar-denote/issues/15.

@localauthor WDYT is the best way to handle this case?

The initial impetus was wanting specific complex name formatting to include in new note titles in denote.

Once we tweak this function, with this commit, it should be easy to do something like this:

((sn citar--shorten-names)
 (etal citar-citeproc-format-reference 
    citar-citeproc-csl-name-only-etal)) ; the latter is a variable for the style

... and then in a template this, which would run that function (though there's a wrinkle here I need to think about more):

"$["author" "editor":%etal}"

[bdarcus]

As you see, I merged a solution, though I confess I don't totally understand what I did (let-binding the argument?)!

If you see any issues, let me know.