Use Readability.php for HTML Parsing

Problem/Motivation

The HTML cleaning up brings in a bunch of stuff that's unrelated to the content you actually want to scrape.

Steps to reproduce

1. Visit the Explorer
2. Select that you'll be reading in a website
3. Select Webpage Docuemnt Loader Plugin
4. Enter in a blog post like https://www.kalamuna.com/blog/why-kalamuna-endorses-un-open-source-principles
5. Output to Markdown
6. Click Load
7. Expect to see just the content you care about, and nothing else

Proposed resolution

Readability.php is a PHP port of Mozilla's Readability to extract the readable version of HTML. It does a really good job on parsing and extracting, and bundling up.

Recommend bringing that in for parsing the HTML so that content is focused on what's actually desired.