Allow permalinks to use string interpolation

[goncalotomas]

Currently permalinks override the default behaviour and allow writing to a custom path.

We can extend this further by allowing permalink to use string interpolation:

---
permalink: "my-custom-path/#{DateTime.from_iso8601(date).year}-#{title}"
---

Allowing permalinks to use interpolation will be needed to implement pagination, since rendering many pages from the same template should allow for customizable paths too.