Custom icon for folder notes

[p2kmgcl]

Since we are already using a real file as a folder note, it would be nice to read some properties from the file to customize how the folder note looks (thinking about Notion's UI).

My proposal is to read this information from the folder note:

---
icon: 🗻
---

and use it as icon on the file tree.

Do you think this is a good idea? Would you accept a PR with this feature?