Allow customJS for posts via metadata

[niklas88]

This change allows adding custom Javascript to posts in a way analogous to the global site.

To add a Javascript file to a post one only has to add a customJS option to the metadata like so

---
title: "Code Test"
date: 2018-03-28T12:32:57+02:00
draft: false
customJS:
     src: "js/myscript.js"
---
... rest of post ...

I'm not really happy with the code duplication but when I delete the {{ range .Size.Params.customJS }} site wide customJS breaks. So I'm not sure if there's a better way. Also of course this still lacks documentation but I wanted to get a discussion started before writing that.

[kakawait]

@niklas88 Thank you for contribution

I think it will be nice to do the same thing for customCSS.

Do you think you can add that addition?

Else no problem I will merge it like that and add after