[YT] Add support for channel URLs without /c/

[someziggyman]

Checklist

• I'm reporting a site feature request
• I've verified that I'm running youtube-dl version 2020.12.22
• I've searched the bugtracker for similar site feature requests including closed ones

Description

Looks like Youtube channels can be accessed via a "simpler" URL that does not have /c/ stage in the link:
https://www.youtube.com/zsecurity

Regular link:
https://www.youtube.com**/c/**PeterSantenello

Right now pasting non- /c/ URL results in falling back to Generic extractor and unsupported URL error. Which is expected behavior. Adding support for these channel URLs will be a nice addition.

Here's a solution that can be easily implemented:
https://github.com/blackjack4494/yt-dlc/pull/190/files