Display CHANGELOG.md or changelog.txt if present

[viktorix]

This is for future consideration. If the plugin/theme zip file includes CHANGELOG.md or changelog.txt, we should parse it and show it on the plugin/theme page. It's very helpful in WP repo or any software, we should take advantage of it and display it to give users helpful information.

I don't think we need tabs, we could do something as simple as:

or something like this:

And it could open a modal (which is already built-in) or it could hide README.md and display changelog (sort of like tabs).