Clobbering YouTube auto subs for some reason

[mtcjayne]

I confirm I have:

• Verified that I'm running youtube-dl 2018.06.04
• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
• Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

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If the purpose of this issue is a bug report, site support request or you are not completely sure provide the full verbose output:

youtube-dl --skip-download --write-auto-sub -o "%(title)s" -v https://www.youtube.com/watch?v=v0WQ2NnDuOg
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'--skip-download', u'--write-auto-sub', u'-o', u'%(title)s', u'-v', u'https://www.youtube.com/watch?v=v0WQ2NnDuOg']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2018.06.04
[debug] Python version 2.7.13 (CPython) - Linux-4.9.0-6-amd64-x86_64-with-debian-9.1
[debug] exe versions: ffmpeg 3.2.10-1, ffprobe 3.2.10-1, rtmpdump 2.4
[debug] Proxy map: {}
[youtube] v0WQ2NnDuOg: Downloading webpage
[youtube] v0WQ2NnDuOg: Downloading video info webpage
[youtube] v0WQ2NnDuOg: Looking for automatic captions
[youtube] v0WQ2NnDuOg: Downloading MPD manifest
[youtube] v0WQ2NnDuOg: Downloading MPD manifest
[debug] Default format spec: bestvideo+bestaudio/best
[info] Writing video subtitles to: 20180601160701.en.vtt

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Description of your issue, suggested solution and other information

A video in question is titled 20180601160701.done_002 on YouTube but youtube-dl downloads the auto subs to 20180601160701.en.vtt instead of 20180601160701.done_002.en.vtt as a user (at least this user) might expect.

[dstftw]

Output template must contain unhardcoded ext.