_expand = function( dst, theTheme ) {

  // Set up the destination collection. It's either the array of files passed
  // by the user or 'out/resume.all' if no targets were specified.
  const destColl = (dst && dst.length && dst) || [PATH.normalize('out/resume.all')];

  // Assemble an array of expanded target files... (can't use map() here)
  const targets = [];
  destColl.forEach(function(t) {
    const to = PATH.resolve(t);
    const pa = parsePath(to);
    const fmat = pa.extname || '.all';
    return targets.push.apply( targets,
      fmat === '.all'
      ? Object.keys( theTheme.formats ).map( function( k ) {
        const z = theTheme.formats[k];
        return { file: to.replace( /all$/g, z.outFormat ), fmt: z };
      })
      : [{ file: to, fmt: theTheme.getFormat( fmat.slice(1) ) }]
    );
  });
  return targets;
}
Expand output files. For example, "foo.all" should be expanded to
["foo.html", "foo.doc", "foo.pdf", "etc"].
@param dst An array of output files as specified by the user.
@param theTheme A FRESHTheme or JRSTheme object.