Allow arbitrary attribute syntax in class and ID names

[lambda-fairy]

Currently, all kinds of syntax is allowed in attribute values:

a href="pinkie.html" { /* ... */ }
a href=(pinkie_url) { /* ... */ }
a href={ (PINKIE_ROOT) "/pinkie.html" } { /* ... */ }
a href=@if pony == Pony::Pinkie { "pinkie.html" } @else { "maud.html" } { /* ... */ }

It would be nice if this syntax is allowed in class and ID shorthand as well:

p."pinkie-123" { /* ... */ }
p.(pinkie_class) { /* ... */ }
p.{ (CLASS_PREFIX) "-pinkie" } { /* ... */ }
p.@if pony == Pony::Pinkie { "pinkie" } @else { "maud" } { /* ... */ }

This would neatly cover the use cases in #45, #98, and #105.

[eedahl]

I foolishly opened a duplicate of this issue #130. I believe it offers a compelling use-case where what seems to be the current short-hand is somewhat unfeasible.

[lambda-fairy]

I've implemented this on a local branch. I'll push it up when I get WiFi on my laptop.