Detects the value of an option at the tree node.

@param {node} ast
function(ast) {
    let detected = [];

    ast.traverse(function(node) {
      if (!node.is('block') && !node.is('atrulers')) return;

      if (node.first().is('space')) {
        detected.push(node.first().content);
      } else {
        detected.push('');
      }
    });

    return detected;
  }