Adjustable runtime Parameters

I was working on trying to get a nice looking [fibonacci tree][1], and I had two constants
```
branchAngle = 30
splayAngle = 30
```
which I keep tweaking, but it's hard to get them right. It'd be pretty neat if I could mark them as "Parameters" or something, and have sliders for their values to adjust and see what happens in realtime, instead of recompiling all the time.

But I'm not sure how it'd look for the code. I mean, it could just be a reader monad, `(Parameters -> Picture)` kinda thing, but that seems like a massive change to the fundementals to have to write everything in monadic style now.

[1]: https://code.world/#PbBOAouMSCYKTj4rseeuyZg

Comienza con el ejemplo enlazado del árbol de Fibonacci y el comportamiento actual del runtime y el editor de CodeWorld. Determina cómo deben declararse los Parameters definidos por el usuario, exponerse como controles deslizantes y aplicarse durante la ejecución en tiempo real sin requerir una recompilación. Se considerará terminado cuando el diseño esté acordado y un ejemplo funcional demuestre valores ajustables.

Escrito por el modelo de indexación a partir del texto del issue.