make addStyleToHead optional to easily comply with Content Security Policy

[dazer]

When setting some strict CSP directive (like "default-src 'self'"), <style> tags are rejected.
I suspect a sane approach to this -without whitelisting, nonce or hash- would be to optionally call the method on the first call to inject, rather than on creation?. The css to prevent image flash would need to be included in an external css, which I feel is fairly straighforward.
Or maybe there's some other approach I'm not seeing.