Function plugin

test/api.js:22–32

(tree)

    test('<a></a><a></a><a></a>', '<c></c><c></c><c></c>', plugin, {}, done)

    function plugin (tree) {
      tree
        .walk(node => node)
        .walk(node => node)
        .match({ tag: 'a' }, () => ({
          tag: 'b'
        }))
        .match({ tag: 'b' }, () => ({
          tag: 'c'
        }))
    }
  })

  it('walk', done => {