periscope: use username instead of user_id for uploader_id?

[Vrihub]

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Currently when downloading from periscope.tv, uploader_id reports the user_id JSON name, which is a string of random alphanumeric characters; maybe it would be more meaningful to use the username JSON name instead, which is the username that appears in periscope.tv URLs (and maybe this would be consistent with youtube uploader_id?)

[dstftw]

For meaningful name use meaningful field uploader.