save methods

Adding three methods to the API:

* saveTable()
* saveJSON()
* saveStrings()

As part of this process, I also worked on the PrintWriter class, which formats output, the writeFile() method, which prompts download, and some more helper methods like escape(). I hope these'll come in handy down the line !
to match stylistically with methods like loadImage, createGraphics, etc, let's say
`this.pWriters.name = new p5.PrintWriter(name, extension);`