Remove css check or just throw a warning on invalid CSS rules.

[OlavHN]

Thanks for a great lib!

So far I've run into missing css rules for 'display: flex' and 'filter: XX'. Just an omission in chenglou/valid-css-props or is there a set of supported browsers?

If it would just throw a warning it would make it easy to send a PR and wait for the warnings to disappear instead of having to use a fork.

[chenglou]

(Related: #56)

Display flex definitely works. Filter is an omission. I forgot where I took the list but it probably didn't include it because it's experimental (https://developer.mozilla.org/en-US/docs/Web/CSS/filter). No ie support. But hey, who am I to say which browser you should support =)

Maybe it should be added to valid-css-props. You can submit a PR there and I'll make sure to sync it back here asap. Using a warning certainly seems to make sense in the future.