Support RSS transcripts

[fulldecent]

https://help.apple.com/itc/podcasts_connect/#/itcb54353390

<podcast:transcript> | A link to the episode transcript in the Closed Caption format. Apple Podcasts will prefer VTT format over SRT format if multiple instances are included.You should use this tag when you have a valid transcript file available for users to read. Specify the link to your transcript in the url attribute of the tag. A valid type attribute is also requried. Learn more about <podcast> namespace RSS tags on the Github repository.Options for displaying transcripts are available in Apple Podcasts Connect for each show. Learn more. | <item>
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<br class="Apple-interchange-newline"><podcast:transcript>

A link to the episode transcript in the Closed Caption format. Apple Podcasts will prefer VTT format over SRT format if multiple instances are included.

You should use this tag when you have a valid transcript file available for users to read. Specify the link to your transcript in the url attribute of the tag. A valid type attribute is also requried. Learn more about <podcast> namespace RSS tags on the Github repository.

Options for displaying transcripts are available in Apple Podcasts Connect for each show. [Learn more](https://podcasters.apple.com/support/5316-transcripts-on-apple-podcasts).

<item>