Make add_theme_support required if tag exists

[HardeepAsrani]

Right now theme check shows the following error if the theme doesn't contain: add_theme_support( "post-thumbnails" );

RECOMMENDED: No reference to post-thumbnails was found in the theme. If the theme has a thumbnail like functionality, it should be implemented with add_theme_support( "post-thumbnails" )in the functions.php file.

That's fine for the some themes. But if a theme contains featured-images tag in its style.css file, then it should be required.

Same goes for post-formats, editor-style, custom-background, custom-header and a lot of tags.

Hope it's valid idea. :)

[grappler]

Yes, we started the discussion at #128

[grappler]

Looking at the code, it seems like an old check to see if the theme was using a custom solution for featured images. https://github.com/Otto42/theme-check/blob/master/checks/postthumb.php

I suppose we could remove this check now as it is not likely to come up. Post Thumbnails was introduced in WP 2.9