Add Opera viewport token
Add Opera viewport token

Seeing 

```
Unknown @ rule: @-o-viewport. This rule looks for recoverable syntax errors. (errors)
```

when parsing some CSS.
medikoo/parser-lib,ash1982ok/parser-lib,malept/parser-lib,malept/parser-lib,cscott/parser-lib,ash1982ok/parser-lib,ovanderzee/parser-lib,ideadapt/parser-lib,medikoo/parser-lib,cscott/parser-lib,ideadapt/parser-lib,ovanderzee/parser-lib