[ADD] Commands: Added QUIT command (as alias to EXIT)

Very, very, very often I type 'quit' to 'exit' BaseX on the command line.
I guess this is because 'quit' or 'q' is often used in Unix commands (vim(1), for instance, accepts both and I use ```:q``` a lot).
I'm feeling very depressed when BaseX is answering:
```
> quit
Stopped at , 1/5:
Unknown command: quit. Try HELP.
```
I thought, maybe it's this pull request that could improve my personal experience?

I like a charming:
```
> quit
Enjoy life.
```