Changes to attr in rules parser

[bcardarella]

We want to more closely align with CSS's own format: https://developer.mozilla.org/en-US/docs/Web/CSS/attr

1. attr(foo-bar) should be supported without quotes. In fact we want to drop support for the quoted version
2. attr(foo-bar, "default") should use the 2nd argument as a default value if the first attribute results in an empty string