fix: import error on date title

[leangseu]

When I try to import markdown with date format title, the parsing fail. js-yaml will always convert the title into the date format. The caveat of this approach is the create_date and update_date will also become string. However, since we are not importing that, it doesn't seem to be an issue for importing.

An alternative approach is to make sure title is a string. However, I don't want my 2023-01-01 turn to a long formatted date.

You can see this by pasting the code below https://nodeca.github.io/js-yaml/

---
title: 2023-01-01