Expose intersectionRatio value to callback and render fns

[jamesism]

What do you think about exposing the intersectionRatio returned in the latest IntersectionObserver callback to the render function and onChange callback? Pretty small change, gives support for a different set of use cases.

Let me know what you think, and if you're down I can update README etc as well before merge.

[coveralls]

Pull Request Test Coverage Report for Build 453

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.783%

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Totals
Change from base Build 444:	0.0%
Covered Lines:	81
Relevant Lines:	81

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💛 - Coveralls

[thebuilder]

That would be great.👍

I haven't had the need for it, but it does makes sense to receive the ratio when working with an array of thresholds, so you know the trigger point. Keep in mind that the ratio won't match the threshold values, so you can't directly compare the values.

The .d.ts file should also be updated with the new interface.

[steinbergh]

This being pulled in with updated types is exactly what im looking for

[thebuilder]

Made some local changes to the typescript definition, and added it to a new pull request.