Add support for audio captions

[drtheuns]

Add support for !(caption)["url"].

See:

• https://github.com/speechmarkdown/speechmarkdown-ex/pull/4
• https://github.com/speechmarkdown/speechmarkdown-test-files/pull/5
• https://github.com/speechmarkdown/docs-speechmarkdown-reference/pull/6

[arjan]

@rmtuckerphx any chance you could look at this PR?

[rmtuckerphx]

@arjan I made you a maintainer. Go ahead and make updates to this library. Please continue to add tests to new code. Best wishes.