checkForBlock = function (node, cb) {
  if (node.contains('block')) {
    curLevel += 1;
    node.forEach('block', function (block) {
      traverseBlock(block.content, cb);
    });
    curLevel -= 1;
    parentSelector.pop();
  }
}
Checks if a node contains a block and if so calls our traverseBlock method. Also
handles our current level counter.

@param {object} node - The current node / atRule part of our selector
@param {object} cb - The callback function we wish to pass through
@returns {undefined}