Add possibility to generate custom urls from markdown

[cedricdelpoux]

Hi,
In phenomic 0.X it was possible to generate custom urls from markdown front-matter.
In phenomic 1.X, it is not possible. The generated url depend of the folders arborescence.

For exemple for a file content/posts/post1.md

---
title: Post 1
date: 2017-01-01
url: my-custom-url-post-1
---
# This is a my Post 1 content

I want the post to be generated here : `dist/my-custom-url-post-1/index.html