Refactor react components so JSNES can run a single ROM standalone

[bfirsh]

A common use case for JSNES is by homebrew ROM authors to run demonstrate the ROM on their websites.

This is currently quite hard to do - see bfirsh/jsnes#219. It'd be really nice if jsnes-web was refactored somehow so that it could be embedded to play a single ROM.

[bfirsh]

This was mostly done here: #170

The only thing remaining is to release this (or jsnes-react?) as a package on npm.