feat: support extracting datetime from post filename

[chawyehsu]

I don't know how to add test case, so I put it here.

saber-config.yml

permalinks:
  post: /:year/:month/:day/:slug.html

./pages/_posts/2019-03-24-hello-saber.md

---
title: Hello Saber
date: 2019-03-25
layout: post
---

Hello Saber!

Permalink should be /2019/03/25/hello-saber.html

./pages/_posts/2019-03-26-saber-post.md

---
title: Saber Post
layout: post
---

Eos eu docendi tractatos sapientem, brute option menandri in vix, quando vivendo accommodare te ius. Nec melius fastidii constituam id, viderer theophrastus ad sit, hinc semper periculis cum id. Noluisse postulant assentior est in, no choro sadipscing repudiandae vix. Vis in euismod delenit dignissim. Ex quod nostrum sit, suas decore animal id ius, nobis solet detracto quo te.

Permalink should be /2019/03/26/saber-post.html

Should close #31

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[egoist]

that should work, I will figure out a way to add unit/e2e tests later when api gets more stable.