Addresses #393
** Current behaviour
Currently, preview selection is limited to a set of choices: first, random, and spread

** Changesintroduced by PR This change allows for custom code to be used for selection of which images to use for generating previews for albums. It works similarly to thealbums-fromargument, changing thealbum-previewsargument to take afile://` url and defer to custom code in that file.

** Does it introduce a breaking change for existing users?
It does not break existing code. Arguably, it makes the error slightly more opaque for users who put in something wrong, since choices is more specific than string, but it will not break any existing workflow using correct values.