[audible] Add new extractor

[triple-j]

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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

This pull request adds support for Audible.com, which is a store where audiobooks can be purchased. This code will download an HLS stream used by Audible's web based Cloud Player. This code also allows the download of a user's entire library of audiobooks via a playlist (AudibleLibraryIE). When not logged in the code can only scrape limited data (title, author, performer, series, description) about an audiobook as well as download an audio sample, but when logged in more data (performance type, categories, release date, etc.) is available as well as the full audiobooks.

I was unable to automate the login process (I was stopped by a CAPTCHA), so one must login via a browser and export one's cookies to gain access. I've tested the cookie method (via the --cookies argument) both with an old Audible account and new Amazon account, both work.

I also added an HTML-to-markdown converter. My reasoning is that some audiobooks, like When Will Jesus Bring the Pork Chops? By: George Carlin, have complicated descriptions with unordered lists, multiple headings, and text emphasis. I wanted to keep as much of the formatting as possible as well as allowing the description to still be readable as plain text.

The modification to _extract_akamai_formats was to prevent the F4M failure warning when extracting the HLS data. I used _extract_wowza_formats as inspiration for the change.

[dstftw]

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