Select field: add option to select multiple items

[cloetensbrecht]

It would be nice to add an option to select multiple items within the select.

{
  multiple: true
}

When this option is present the json data should be an array containing the selected items.

{
  ...
  "favouritefood": ["pasta", "pizza"]
}

The initialValue option should also allow an array to pre-select these items.

Playground:
https://alinea.sh/playground#code/aW1wb3J0IHsKICBzZWxlY3QsCiAgdHlwZQp9IGZyb20gJ2FsaW5lYScKCmV4cG9ydCBkZWZhdWx0IHR5cGUoJ1R5cGUnLCB7CiAgZmF2b3VyaXRlZm9vZDogc2VsZWN0KCdJIGxpa2UgdG8gZWF0JywgewogICAgICAncGFzdGEnOiAnUGFzdGEnLAogICAgICAnZnJlbmNoZnJpZXMnOiAnRnJlbmNoIGZyaWVzJywKICAgICAgJ3BpenphJzogJ1BpenphJwogIH0sIHsKICAgIG11bHRpcGxlOiB0cnVlCiAgfSkKfSk=