Add functionality to exclude certain tables

The way I imagine this to work is with a flag `--exclude` that takes a comma-seperated (?) list of strings to exclude the tables you like to exclude. For example:

```command
pg-erd --exclude users,logs foobar_database
```

Or it would be possible to use the `--exclude` flag more than once:

```command
pg-erd --exclude users --exclude logs foobar_database
```