Automatically save previous distro when updating

previous disto = "^distro"

I'm not sure about the ^ notation. Since you have to quote it in the shell, for example:

```
zinc distro:delete -c videos -b English-Spanish-level31 -d ^test
```

does not work (in zsh at least).

you have to do

```
zinc distro:delete -c videos -b English-Spanish-level31 -d '^test'
```

Any suggestions for a better character?