function walk (cb) {
  return traverse(this, cb)
}
Walks the tree and passes all nodes via a callback

@memberof tree

@param  {Function} cb  Callback
@return {Function}     Callback(node)

@example
```js
export const walk = (tree) => {
  tree.walk((node) => {
    let classes = node.attrs && node.attrs.class.split(' ') || []

    if (classes.includes(className)) return cb(node)
      return node
  })
}
```