[Feature] JSXGraph

[joyqat]

Add new js library JSXGraph.

JSXGraph is a cross-browser library for interactive geometry, function plotting, charting, and data visualization in a web browser. It is implemented completely in JavaScript, does not rely on any other library, and uses SVG, canvas, or even the venerable VML. JSXGraph is easy to embed and has a small footprint: approx. 160 KByte if embedded in a web page. No plug-ins are required! Special care has been taken to optimize the performance. JSXGraph supports multi-touch events and runs on all major browsers, even on very old IEs. JSXGraph is developed at the Lehrstuhl für Mathematik und ihre Didaktik University of Bayreuth, Germany

Functionality was implemented using jessiecode DSL used by JSXGraph. See DSL document for details.

TODO list:

• [x] add library
  • [x] basic functionalities
  • [x] ~~style for different themes~~ default color seems fine in all 5 themes.
  • [x] ~~responsive width~~ user defined width should be better, need to find a way to implement it.
• [x] add proper examples
  • [x] basic examples
  • [x] adapt some examples from https://github.com/jsxgraph/JessieCode/tree/master/examples
• [x] add document

---

Specify size of graph:

```jessiecode:400x300
// codes...
```

size of graph will be width 400 and height 300.