Should be able to define the where <li>s should be inserted

[wilsonpage]

When working with shadow-dom we will likely want to insert list items into the 'light-dom' but keep list structural elements hidden in the shadow dom. For this to be possible we'd need to be able to write something like:

new FastList({
  container: this.shadowRoot.querySelector('section'),
  insertItemstInto: this,

  ...
});

[wilsonpage]

Fixed by b870b66