Adding support for media query specificity bumps

[src-code]

Implements #384.

Introduces a new option to Atomizer, --bump-mq:

  --bump-mq                           Increases specificity of media queries a small amount

For --bump-mq I'm relying on an attribute selector, [class], which we know will always match (since the Atomic classname itself is applied via this attribute.) This raises the specificity by 0,0,1,0.

cc @snyamathi

[thierryk]

👍 for:

--bump-mq                           Increases specificity of media queries a small amount

👎 for:

--bump-mq-important                 Override specificity of media queries by applying !important

I think we should offer the minimum bump we can without introducing a new "specificity context". Our use of !important is justified with descendant selectors but I don't see what justifies its use here.
What problem does --bump-mq-important solve that --bump-mq does not?

[src-code]

Sure, I can remove the support for !important for now, and if we determine that there's a valid use case, we can bring it back later.