Added the ability to generate outlines for PDFs

--outline-tags allows to specify the HTML tags which should be considered for the outline. The tags are expected to be given in order of hierachy, for example, 'h1,h2' will trigger a generation with h1 elements as top level outline entries and h2 as their childs.

Ideally this would not be required if Chromium would add this directly. So if these bugs are closed this can probably be removed again:

• https://bugs.chromium.org/p/chromium/issues/detail?id=840455
• https://github.com/GoogleChrome/puppeteer/issues/1778

This code is heavily based on @Hopding's comment at: https://github.com/Hopding/pdf-lib/issues/127#issuecomment-502450179

bin/paged

@@ -26,6 +26,11 @@ program
   .option('-t, --timeout [ms]', 'Set a max timeout of [ms]')
   .option('-x, --html', 'output html file')
   .option('-b, --blockLocal', 'Disallow access to filesystem for local files')
+  .option('--outline-tags [tags]', 'Specifies that an outline should be ' +
+          'generated for the resulting PDF document. [tags] specifies which ' +
+          'HTML tags should be considered for that outline. ' +
+          '"h1,h2" will trigger an outline with "h1" tags as root elements ' +
+          'and "h2" elements as their childs.')
   .parse(process.argv);
 
 
@@ -122,6 +127,7 @@ if (typeof input === "string") {
       file = await printer.html(input, options);
       output = replaceExt(output, '.html');
     } else {
+      options.outlineTags = !program.outlineTags ? [] : program.outlineTags.split(',');
       file = await printer.pdf(input, options);
     }
   } else {