Rewrite loader to be asynchronous

* Insert `note_embed.js` using `document.createElement` instead of `document.write`, and flag it `async`
* Only write one copy of `note_embed.(js|css)` to page
* Queue up notes to execute after `note_embed.js` is availble
* Add nuclear option for creating the target DIV for a note that has an inline script but no container

# Conflicts:
#	dist/note_embed.js

Author: reefdog