Feature: Support downloading multiple subtitle formats of the same language at once

[altimumdelta]

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

I first thought this is a bug, but then I changed my mind based on some educated guessing, I may be wrong.

I have manually checked and ttml is downloadable fine from the YT videos I'm downloading in this case.

--all-subs doesn't seem to do it, it apparently applies only to "all languages" in this case it's only english, and it takes a format it thinks it's most appropriate, vtt.

--sub-format only takes in one valid format or a preference, it does not support comma separated multiple values.

These options should be renamed as they feel too vague and it's not clear whether --all-subs should be used with --write-sub or not, lack of context.

It could be --sub-formats to reflect it could accept commas, or a new option --all-sub-formats
Additionally --all-subs should be renamed to --all-sub-langs , but it's already inconsitent because I found out --all-subs doesn't need --write-sub so from the name it seems as an optional secondary option but it is infact a standalone command which means it should be called --write-all-sub-langs, however I thought that "write" meant baking it into the file with ffmpeg or even into the video it self, maybe get or save could be a better term, but it's not that big deal.

However it should be named appropriately so that it wouldn't conflict with exactly that idea of merging/packing the subs into a supported container described here: #21344
Still, those terms would rather be "pack" "merge" so it may not be an issue.

--
Currently it's only practically possible downloading other subtitle formats in a separate session with
--skip-download option.

[RevSnowfox]

I vouch for this, it would be nice to be able to download several formats of subtitles, most importantly YouTube's own SRV3 format for archival purposes.