allow custom css to be applied directly to SCORM packages

Some SCORM packages contain strange css/hard-coded layout that cause the package to be aligned strangely or to display incorrectly - I've been thinking about adding the ability to inject CSS directly into the SCORM iframe to help fix these issues.

It would be good to hear if others think this is a good idea too! - I'll attach a hacky proof-of-concept that I'm playing with at the moment.
https://github.com/danmarsden/moodle/commit/be2af4cb033bd469b94d324ad9e7724fc11704cf