Apt improve

I don't have an environment to test this in right now, but would find the features very helpful.

Can be tested with a simple sls like the following:

```
hold-us:
  pkg.hold:
    - grub-pc
    - linux-image-generic

```

It should return a list of `{ <package>: True }` dicts, or an error, or, if nothing was specified, an empty list.

After applying the state, they should be printed when the following command is run:

```
dpkg --get-selections | grep 'hold$' 
```

Changing `hold` to `unhold` in the sls should make the packages no longer show up in the shell command.

I'll update this req if I get the chance to test it.