Adding 'next' command to workshopper

Currenty, when a student finishes an exercise, the only way to print instructions for the next exercise is to fire up the workshopper menu, navigate down with down arrow key to the next exercise, and press enter.

The `next` command provides an easy way to quickly navigate to the next exercise:

```
appname next
```

This will print the instruction for the next incomplete exercise and updates workshopper's "current" exercise reference internally.