Add a semantically meaningful way to open a PostgreSQL console.

This PR adds the ability to open a `psql` console to the specified database container with the command 
```
dokku postgresql:console <db>
```

I was looking for a way to do this and for a while I would just manually connect with `psql -h 172.17.42.1 -p 49153 -U root db`. Then, I saw that the `postgresql:restore` command does exactly that when you don't provide the dump file with `< dump_file.sql`.

Since this didn't feel very correct in a semantic way, I added the feature to achieve this with the `console` command mentioned above.

The README has been updated as well to show the newly added command.