Add postgres pg-psql command


```
✗ ./hk pg-psql -a hkdist -c 'SELECT 1;'       
 ?column? 
----------
        1
(1 row)

➜  hk git:(pg) ✗ ./hk pg-psql -a hkdist               
psql (9.3.1, server 9.1.11)
SSL connection (cipher: DHE-RSA-AES256-SHA, bits: 256)
Type "help" for help.

d1234567890abc=> 
```

This probably needs a bunch of cleanup. Ultimately we'll want to refactor the "run this SQL command" bit into its own function that we'll reuse from lots of other commands.

Would like to take STDIN ideally too.