Added $overwrite attribute

I forked your branch instead of the original because it fixed the issue with the CACHE_* defines.

This fix adds an $overwrite attribute (default true) which allows the user to switch whether pre-existing rules in style attributes will be overwritten or not. It defaults to "true" since this is backwards compatible.

Setting it to false should produce more browser-like behaviour, where in-line styles overrule CSS styles. Not having the rules be overwritten would produce more accurate results.

Hope you're willing to adopt this change into your branch.

I also fixed a typo in composer.json.