Detects the value of an option at the tree node.

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

    ast.traverseByType('string', function(string) {
      if (string.content[0] === '"') {
        detected.push('double');
      } else if (string.content[0] === '\'') {
        detected.push('single');
      }
    });

    return detected;
  }