Updated old and added new IMG extensions. (#90)

* Added IMG HTML extension files.

* Redirect previously published IMG extensions to their new HTML files.

* Added extension 66: cl_img_generate_mipmap.

* Added extension 67: cl_img_mem_properties.

* Updated clext.php with update IMG extensions.

* Alphabetise IMG extension ordering.

* Update txt versions of IMG extensions to signpost the HTML version.