Support class grouping

[juice49]

I'd like to support class grouping, similarly to that described by Harry Roberts.

Based on Harry's example:

<div class='[ foo  bar ]  baz'>

Would compile to:

<div class='foo foo--bar baz'>

My current idea is to take the first class from the group and use it as the block/element qualifier, with all following classes in the group being modifiers.

This would be an opt in compile mode. I don't think the group syntax needs to be configurable; I'm inclined to just use [] for simplicity.