Dev

* cd called without argument crashed - now cds to /

* added output redirection operator to cat
* added output redirection operator to echo

* ls now prints only visible files
* ls -a prints all files, including hidden

* added micropython command
It can execute scripts on the board and it has an interactive REPL. However, it uses exec(), which normally isn't a good idea, but I don't think this applies here.

Note that commit 20e2d0c will not work without 99ea6e5