Add aria-live-tag to slides

[LangdalP]

I was testing how well a screen reader would work with remark.js slides. In general, it works quite well, but the screen reader does not catch that the content of a slide changes when the user clicks the left or right arrows. I have fixed this by adding an ARIA live region. This gives a hint to screen readers that it should listen for changes inside the region, and read these aloud.

I have tested this change with Mac OSX's VoiceOver utility. The change works as intended. However, I am not sure if it is possible to write any sensible automated tests for this, so I have not added any tests.

Note that after modifying src/remark.html and running make.js, I observed that two other files had changed. I assume these two files should also be a part of the check-in, since they are not gitignored. Please let me know if that is wrong.

[venkateshpotluri]

Thank you for making this PR! as somebody who uses a screen reader, having this changed merged in would be super helpful, and give presenters who use screen readers one more reason to adopt remark.

[tripu]

@LangdalP: thank you for contributing! I hadn't seen this, I'm sorry. Would you mind sync'ing your fork and resolving the conflicting file, so we can review your changes? Thanks again!