Shared State

I've been wondering if we should have a syntax for creating a state that is shared across all classes.

An example use case for this would be a the `gutter` and `last` states for a float-based grid block.

Using Sass, you can create mixins to simplify this process, but I'd like the block syntax to be usable without a preprocessor.

```css
/* example.block.css */
.foo {
margin: 10px;
}

.bar {
margin: 18px;
}

*[state|last] {
margin-right: 0;
}
```

Would then compile to:

```css
.example__foo { margin: 10px; }
.example__bar { margin: 18px; }
.example__foo--last,
.example__bar--last { margin-right: 0; }
```

Maybe it's not common enough to warrant addressing with specialized syntax, but I thought I'd write it down for discussion at some point.

The issue provides example.block.css as the entry point; start by comparing its block and state syntax with the requested compiled CSS output. Done would require a decided syntax and an implementation that compiles shared states across classes, but the issue does not name tests or implementation files.

Written by the indexing model from the issue text.