Metalsmith Plugin that collates a list of pages that are to be included in the site navigation
into the metadata, along with their URLs.

If the metadata has a `sites.pages`, then it is expected to be an array of URLs which indicates
the ordering to be applied; `sites.pages` is replaced with an array of objects, one per page,
that contains `url` and `title` properties.
function getPages(files, metalsmith, done) {
  var metadata = metalsmith.metadata();
  
  var pages = [];
  var order = {};
  if (metadata.site.pages)
    metadata.site.pages.forEach((url, index) => typeof url == "string" ? order[url] = index : null);
  var unorderedPages = [];
  
  function addPage(url, title) {
    var page = {
      url: url,
      title: title
    };
    var index = order[url];
    if (index !== undefined)
      pages[index] = page;
    else
      unorderedPages.push(page);
  }
  
  for (var filename in files) {
    var file = files[filename];
    if (filename == "index.html")
      addPage("/", file.title||"Home Page");
    else if (file.permalink || file.navigation)
      addPage(file.permalink||filename, file.title||"Home Page");
  }
  
  unorderedPages.forEach(page => pages.push(page));
  metadata.site.pages = pages;
  
  done();
}