Add link preview component

Fixes #3

This PR adds a new link preview component that can be embedded to posts via:

{% include linkpreview.html url = "https://genicsblog.com" title = "Genics Blog: An open source developer publication" %}

Where url is a mandatory field and title is optional.

There's an additional rel attribute (optional) that specifies the link type for the preview. By default its value is nofollow noopener noreferrer.

Sometimes, websites dynamically change the <title> tag on page load, through javascript. In that case, if title prop is passed to the include specifically, it is preferred over the extracted <title> tag from the website.

Preview (desktop):

Preview (mobile):

genicsblog / theme-files

Add link preview component #55