API to return whether a selector has non-standard syntax.

[chriseppstein]

1. add a strict parse mode so that any non-standard syntax is an immediate error.
2. When not strict, it should be possible to find which nodes contain non standard syntax and for complex nodes like Attribute, which part(s) are violating the css spec.

[alexander-akait]

@chriseppstein Also maybe we can support non standard comments //?

[chriseppstein]

I'd rather postcss filter that out so that it's not part of the rule's selector value.

[chriseppstein]

postcss/postcss#1149

TIL about postcss-scss. I think the selector parser and tokenizer should be updated to have a syntax option and then the parser should non-standard syntax correctly or error out based on the current syntax setting. Right now it's all mashed together and unpredictable.