Allow for instances of Axolemma

[seanohue]

Use case: Running a single Ranvier server, wanting to create multiple Axolemma "states" and generate new areas accordingly.

Example:

const MazeGenerator = new Axolemma(mazeOptions);
const CavernGenerator = new Axolemmea(cavernOptions);

Make it newable and have it use the instance options as defaults, taking priority over config files but not over the options passed to generate.