Add option to bundle collection into html

[thangngoc89]

This PR add an option to bundle collection to html

ref #92 #96

Usage:

// index.md

---
layout: Homepage
title: Home page
collection: true

---

I'm replying on Travis for test right now, because ava freezes my laptop

[MoOx]

I think this should be default behavior. I don't want a weird "collection" option in meta (and you don't want neither, am I right?).
Maybe a "collection: false" to disable in frontmatter, and a collection: true by default in js config?

Also you will need to adjust the useless fetch for the collection.json file if the collection is already in the initial state.

[MoOx]

Did you try "ava src/ a single file.js" to run test?

[thangngoc89]

Oh yeah. you are right. this should be the default behavior.

[thangngoc89]

Oops. Accidentally delete this branch. Continue as #100