Bug when '$PATH' string contained in Video Title

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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 as follows:

Add -v flag to your command line you run youtube-dl with, copy the whole output and insert it here. It should look similar to one below (replace it with your log inserted between triple ```):

$ ./youtube-dl -v "https://www.youtube.com/watch?v=rJMFxIbDe-g"                           
[debug] System config: []                                                         
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'https://www.youtube.com/watch?v=rJMFxIbDe-g']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.07.09
[debug] Python version 3.6.1 - Linux-4.12.0-gentoo-x86_64-Intel-R-_Core-TM-_i5-3210M_CPU_@_2.50GHz-with-gentoo-2.4.1
[debug] exe versions: ffmpeg 3.3.2, ffprobe 3.3.2
[debug] Proxy map: {}
[youtube] rJMFxIbDe-g: Downloading webpage
[youtube] rJMFxIbDe-g: Downloading video info webpage
[youtube] rJMFxIbDe-g: Extracting video information
[youtube] rJMFxIbDe-g: Downloading MPD manifest
[debug] Invoking downloader on
[download] Destination: Everything You Need to Know About /usr/local/bin:/usr/bin:/bin:/usr/x86_64-pc-linux-gnu/gcc-bin/6.3.0:/usr/x86_64-pc-linux-gnu/i686-elf/gcc-bin/5.4.0:/usr/lib/llvm/4/bin:/usr/local/bin:/usr/bin:/bin:/opt/bin:/usr/games/bin in Bash-rJMFxIbDe-g.f137.mp4
[download]   4.9% of 14.43MiB at 71.53KiB/s ETA 03:16^C
ERROR: Interrupted by user

...
<end of log>

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Description of your issue, suggested solution and other information

This issue occured when I was downloading a tutorial video about Bash that contained $PATH in the title. The video URL is https://www.youtube.com/watch?v=rJMFxIbDe-g. When youtube-dl attempts to create the file with the video title to download into the $PATH string in the title actually gets replaced with the contents of the actual $PATH variable in my shell. As you can see by the line in the output showing the destination file this creates a huge mess of directories with the mistitled video sitting at the bottom of it. Videos containing $ are rare but it needs to be escaped, I'm happy to provide any additional information.

Please Note: While I respect the request for the whole output I removed the URL on the line "[debug] Invoking downloader on" as I know it is not relevant to the issue whatsoever but it contained informtion such as my IP address.