Add --jobs option for the convert command

This doesn't seem that helpful as there seems to be a lot of sequential work going on regardless.

lib/worm-scraper.js

@@ -48,6 +48,13 @@ const argv = yargs
     requiresArg: true,
     global: true
   })
+  .option("j", {
+    alias: "jobs",
+    default: 10,
+    describe: "Number of concurrent read/write jobs to run, for the convert command",
+    requiresArg: true,
+    global: true
+  })
   .demandCommand(1) // TODO remove and allow all
   .recommendCommands()
   .help()
@@ -72,7 +79,7 @@ if (argv._.includes("convert")) {
   commands.push(() => {
     return rimraf(chaptersPath)
       .then(() => mkdirp(chaptersPath))
-      .then(() => convert(cachePath, manifestPath, chaptersPath));
+      .then(() => convert(cachePath, manifestPath, chaptersPath, argv.jobs));
   });
 }