Portability/common code

There is now a secret function, `ElmRuntime.debugFullscreenWithOptions(options)` that generates a function with certain features enabled or disabled. With both options enabled, you get the `debugFullscreen` function we both know.
The function `debugFullscreenWithOptions(options)`'s parameter is an object:
```javascript
{
    socket: boolean, // Enable the socket
    hotswapButton: boolean // Show the hotswap button
}
``` 

The js files also now live in a common place, assets/_reactor

This is 97% moved code and a couple added lines:
https://github.com/michaelbjames/elm-server/commit/2d2eb02965b22fe79494a95d837f4eae82f4df69#diff-198999901e12f028670240adef49411bR27 (assets/_reactor/core.js +27)
assets/_reactor/reactor.js