Support media queries in stylesheet package

[lihaoyi]

Currently they don't work. You can do weird workarounds by embedding the entire text of a stylesheet into a string:

tags2.style(s"@media (min-width: 60em) {${styles.Wide.sheet}}"),
tags2.style(s"@media (max-width: 60em) {${styles.Narrow.sheet}}"),

But they really should be supported as part of scalatags.stylesheet