Add support for CSS class names containing the @ sign ("commercial at", U+0040)

[redoPop]

CSS conventions including BEMIT encourage use of the @ sign in CSS class names as part of a suffix indicating the breakpoint at which a class takes effect.

This PR adds Haml support for @ within class names, so we can do:

.o-media@md

Instead of having to do:

%div{class: 'o-media@md'}

A passing test is included.

[k0kubun]

Since @ doesn't conflict with Haml syntax other than Ruby's ivars and the naming is totally valid in CSS, I think there's no problem to have this change. Merging for BEMIT naming convention users.