Feature: Add option to treat batch-file URLs as chunks and merge them

[ghost]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2019.06.08
• I've searched the bugtracker for similar feature requests including closed ones

Description

Sometimes it is possible to download the previous chunks of a m3u8 livestream by specifying a custom m3u8 file or feeding all the chunks in a URL batch file.

In this case I didn't want to exactly start from the beginning of the stream's rewind, which is 2 hours, but just a portion somewhere in the middle, anyhow it seems that the rewind isn't even tied to the raw content files so they may still be up even after the web-player's 2 hour limit.

I tried the batch-file method with the --merge-output-format option but it didn't work, it downloaded fine, I just need a manual concat, not sure about that yet.

Through the course of figuring out this issue, I saw that the m3u8 chunklist file contained all the relative URLs back to where the 2 hour rewind limit is, containing the part I wanted, so actually I wouldn't have needed to manually generate all those full URLs for the chunks using LibreOffice Calc (which isn't that hard as it is time consuming) so basically I could just do that and use the custom m3u8, except youtube-dl doesn't support such a thing, becuase all the chunk links are relative...

I even try putting those chunks into full URLs, it didn't work as if it was an URL, and --batch-file just downloaded the same without merge.

Example URL: https://rtvslolive.akamaized.net/hls/live/584144-b/tv_slo1/slo1_720p/chunklist.m3u8