Hierarchy support for migrations folder

[shedar]

For long running projects migrations folder contains a lot of files.
It's not a big problem, because new one are always at the bottom. But opening of the folder with hundred+ files in IDE file browser is not a great experience. Also you rare need to see old migrations.
I'd suggest to add a config option to group migrations by year/month.
something like

2015
- 05
  - 20150502091757_older_migration.js
- 06
  - 20150602091757_faq_model.js
  - 20150607091757_log_import_status.js