Action helper should accept an array of actions

Previously, I had added the ability for alterColumn to be an array, but this was incorrect and misunderstood behavior. Action should accept an array of various actions:

```javascript
{
  type: 'alter-table'
, table: 'users'
, action: [
    { dropConstraint: 'users_pkey' }
  , { addConstraint: { 'users_pkey', primaryKey: 'name' } }
  , { addConstraint: { 'users_stuff_key', unique: 'name' } }
  ]
}
```