Properly escape & character in ttml (xml) subtitles

[Diegus83]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2019.07.16
• I've searched the bugtracker for similar feature requests including closed ones

Description

WRITE DESCRIPTION HERE

I recently downloaded some subtitles from Discovery and they come with a ttml extension, internally being just XML files.

After manually inspecting one that failed to convert to srt using different tools, the culprit was an & symbol not escaped properly &

<p region="pop317" begin="00:07:40.133" end="00:07:45.800"><span tts:backgroundColor="black" tts:color="white">BACK AT MASELLI & SONS.</span></p>

I have no way to verify if the file was malformed on the server or this was an error when youtube-dl wrote the output file but maybe checking for properly escaped XML files is something that can be added to youtube-dl when an XML subtitle is downloaded.

Complete xml file is here https://pastebin.com/pVw1pb7p

[Diegus83]

Is there any information I missed? Or is this request outside the scope of the project?
I would just like to know.

Thanks!