Support for arbitrary audio files via HTTP

[underyx]

Before submitting this issue, please acknowledge that you have done the following:

• [x] I've at least skimmed through the README
• [x] I have checked that I am running the latest version of MumbleDJ (use mumbledj --version when starting the bot or use the MumbleDJ version command in Mumble)
• [x] I have searched through the existing issues to see if my issue has been answered already

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What type of issue is this?

• [ ] Bug report (encountered problems with MumbleDJ)
• [x] Feature request (request for a new functionality)
• [ ] Question
• [ ] Other:

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Description of your issue:

I'll keep it simple: I'd like to be able to !add https://mumble.party/audio_file.mp3 and have it play.

[matthieugrieger]

Thanks for the feature request! I do plan to support this at some point.