convert p5.js overview tutorial to ES6

[lmccart]

https://github.com/processing/p5.js/wiki/p5.js-overview

As a step forward in the transition to ES6 all of the tutorials on http://p5js.org/learn should be updated to use ES6 standard. This will often mean simply switching 'var' to 'let' or in some cases using es6 classes where applicable.

There are tons of resources online for learning about ES6. Here is an interactive guide that covers the biggest features.

[ihsavru]

I'd be working on this! Updated!

[lmccart]

thanks @ihsavru!