app: add export route

Currently, the export route only creates the snapshot (the other side
of in-place import). In the future, the export route can export to a
custom backup config (like import).

src/apptask.js

@@ -598,7 +598,7 @@ function backup(app, args, progressCallback, callback) {
 
     async.series([
         progressCallback.bind(null, { percent: 10, message: 'Backing up' }),
-        backups.backupApp.bind(null, app, { /* options */ }, (progress) => {
+        backups.backupApp.bind(null, app, { snapshotOnly: !!args.snapshotOnly }, (progress) => {
             progressCallback({ percent: 30, message: progress.message });
         }),