Allow user to load the scripts and styles the way they want

[saurabhdaware]

Avoid adding style or script to parent page if the link already exists.

If user already has the css/js mentioned in index page-

<link rel="preload" href="./bundled-css/main.abell.css" />

We will ignore adding this link to head tag from code.

[saurabhdaware]

Added in #115