External state rendering

[streamich]

• [ ] Ability to render placeholder when document is empty.
• [ ] Ability to render placeholder when new block is empty.
• [ ] Ability to render suggestion text to be inserted, at current caret position.
• [ ] Ability to render suggestion text at any position.
• [ ] Ability to suggest fragment to replace the current range selection.
• [ ] Ability to specify a range selection and its renderer, which is not persisted in the document. (Ability to highlight text in document.)