Use an EventEmitter-like library to abstract postMessage/onmessage boilerplate

We make liberal use of postMessage/onmessage to communicate between different JavaScript contexts in web workers and iframes. As we continue to develop live-editor new features and improve performance the number of contexts and the amount of communication between contexts will increase.

In order simplify our message passing code I'd like suggest incorporating https://github.com/kevinb7/poster. It allows you to post/listen for messages on a particular channel. It's well tested and works transparently with both web workers and iframes.

Review the existing postMessage/onmessage communication in the live-editor, including its web workers and iframes. Read the proposed poster library and determine how it would cover the current message-passing boilerplate; the work is done when the project uses a tested abstraction across those contexts.

Written by the indexing model from the issue text.