Pre-released custom plugins for Toast UI Editor

This custom pre-release contains 3 new plugins:

1. abc for music notation
2. alphatex for guitar tabs
3. mathjax for math typesetting

Also fixed

• react-editor to handle ajax-loaded content.
• making this github repo installable via npm install by adding npm prepare script

Usage

This isn't currently published on NPM, so need to use it as a git submodule,
and install with npm install <relative_path>.