Create a built in tutorial

TL;DR the world's simplest tutorial and perhaps a good place to start for teachers. These should be as simple as possible. For example:

```
>>> import tutorial
Welcome to the built in tutorial!

There are 6 lessons:

0 - How to use MicroPython
1 - Using the display
2 - Buttons
3 - Moving the micr:obit
4 - Input/Output
5 - Advanced stuff

tutorial.show(0) displays the first lesson.
tutorial.list() displays this message again.
>>> tutorial(0)
How to use MicroPython

... some text no wider than 79 characters or longer than 20 lines ...
```

There should be a secret lesson 6 (top secret cool stuff) that is only referenced at the end of lesson 5. ;-)
- [ ] How to use MicroPython - a simple introduction to the REPL, Python and the micro:bit.
- [ ] Using the display - scrolling text and showing images.
- [ ] Buttons - waiting for input and reacting as a result.
- [ ] Moving the micro:bit - using the compass and accelerometer
- [ ] Input/Output - plugging things in via the pins
- [ ] Advanced stuff - animations and making sounds
- [ ] Top secret cool stuff - all about the Easter eggs and where to find out more