Support slug field in Markdown front matter

[oliver-pham]

Requirements

• The folder structure of the generated HTML file should follow slug in the front matter
• slug is set to the file name by default
• Validate slug to make sure it is unique

Examples

If the content of example-lyrics.md is:

---
slug: lyrics/rickroll-1
---

We're no strangers to love
You know the rules and so do I

The following file structure should be produced:

dist
│
└─── lyrics
     |                
     └───rickroll-1.html

If the "lyrics/rickroll-1" is already used as a slug by another document, Silkie should display an error message:

warn: Duplicate routes found!
/rickroll-1 is already taken by another document

[oliver-pham]

Resolved by b37f663