Support Polyfills on Dynamically Inserted DOM Elements

[jsmecham]

Adds support polyfilling elements that have been dynamically added to the DOM, by way of MutationObserver. This isn't supported in some older browsers, but it does allow modern browsers to use CSS polyfills (i.e. position: sticky) on single page applications or other dynamically updated applications.

[philipwalton]

Hey @jsmecham, I'm not really maintaining this project anymore as I've found there are way too many gotchas associated with using inline styles to polyfill CSS (since inline styles do not maintain the original cascade order set in the stylesheet).

I've toyed around with different strategies to keep the cascade in tact, but I haven't yet taken the time incorporate them into this project.

I'm happy to merge this PR if it's helpful to you, but in general I wouldn't recommend using this library to polyfill CSS.