support allowing audio content

[jessehall3]

Description

This addition would provide an option for allowing audio content as well.

The use case would be for a feature in Gitlab supporting embedded audio in markdown.

As with Github, Gitlab allows users to provide embedded images in markdown: ![my image](http://example.com/my_image.jpg)

Additionally, Gitlab permits embedded video, using the same syntax: ![my video](http://example.com/my_video.mp4)

As they use go-camo as their proxy for these assets, which now supports video, all of this works.

Just this week, I submitted a merge request to allow embedded audio, not knowing about this constraint as it regards to the go-camo server.

I think it'd be worth supporting this extra format as it would not expose the execution context and would require only a small addition to the current code base.

I would love to hear what you think, thanks.

Checklist

• [x] Code compiles correctly
• [x] Created tests (if appropriate)
• [x] All tests passing
• [x] Extended the README / documentation (if necessary)

[dropwhile]

Nice PR, and this seems like a solid use-case. 👍

[jessehall3]

Wow, that was incredibly fast. Thanks again.