[LaraApi] Graph dot formatter now supports layout options

[tiagolascasas]

Adds a method to specify layout options on a DotFormatter object, and further edits Graphs.js to generate a DOT that includes those options.

Usage example:

formatter.setLayoutOption("rankdir", "LR");
formatter.setLayoutOption("rankdir", "TB"); // overrides previous value
formatter.setLayoutOption("layout", "neato");

Generated DOT:

digraph G {
rankdir="TB";
layout="neato";
...