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

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.

Start by tracing RCSS's CSS validation and its use of chenglou/valid-css-props, then reproduce the handling of `display: flex` and `filter: XX`. Done means the intended behavior is established and unsupported or missing rules no longer unnecessarily block use, either by accepting them or issuing a warning.

Written by the indexing model from the issue text.