'deken shell' for interactive packaging/uploading/...

E.g

```command
$ deken shell
deken 0.9.1
Type "help" for more information.
deken> help

Documented commands (type help <topic>):
========================================
download  help     package    systeminfo  upload
find      install  systemfix  uninstall   verify

deken> package --version 1.2.3 foobar
deken> upload foobar*.dek
deken> exit
$
```