Enhance cascade url to include entire path

[paololazzari]

This is my project:

content/
├── _index.md
└── posts
    ├── _index.md
    └── today
        └── my-first-post.md

3 directories, 3 files

in content/posts/_index.md I have:

---
cascade:
- url: /:sections[last]/:slugorfilename
title: today
url: '/posts/today'
---

with this configuration I can access the my-first-post page at /posts/my-first-post/, but I want to access it at /posts/today/my-first-post/.