Add boilerplate to IDL files with spec info

See: w3c/webref#3 (comment)

Boilerplate is:

// GENERATED CONTENT - DO NOT EDIT
// Content of this file was automatically extracted from the
// "{spec title}" spec.
// See: {crawled url}

Note that we could also easily add the spec date to the boilerplate, but doing
so would mean that IDL files will be updated in GitHub whenever the spec
changes, even when the changes do not touch on the IDL defined in the spec.

src/cli/crawl-specs.js

@@ -374,9 +374,20 @@ function saveResults(crawlInfo, crawlOptions, data, folder) {
     .then(idlFolder => Promise.all(data.map(spec =>
         new Promise((resolve, reject) => {
             if (spec.idl.idl) {
-                fs.writeFile(path.join(idlFolder, getShortname(spec) + '.idl'),
-                             spec.idl.idl,
-                             err => { if (err) return console.log(err); return resolve();});
+                let idl = `
+                    // GENERATED CONTENT - DO NOT EDIT
+                    // Content of this file was automatically extracted from the
+                    // "${spec.title}" spec.
+                    // See: ${spec.crawled}`;
+                idl = idl.replace(/^\s+/gm, '').trim();
+                idl += `\n\n${spec.idl.idl.trim()}`;
+                fs.writeFile(
+                    path.join(idlFolder, getShortname(spec) + '.idl'),
+                    idl,
+                    err => {
+                        if (err) console.log(err);
+                        return resolve();
+                    });
                 delete spec.idl.idl;
           } else resolve();
         }))).then(_ => new Promise((resolve, reject) => {