Enforce _posts/FILENAME.md filename must have 3 slugs at least.

[huan]

Valid:

• _posts/YEAR-MONTH-DATE-slug1-slug2-slug3-slug4.md
• _posts/YEAR-MONTH-DATE-slug1-slug2-slug3.md

Invalid:

• _posts/YEAR-MONTH-DATE-slug1-slug2.md
• _posts/YEAR-MONTH-DATE-slug1.md

By adding this rule to the CI tests.