Add Support for creating and dropping SQL views

Add support for creating and dropping views. In a migration, a view can be created using a rails relation or literal sql

```ruby
create_view :posts_commented_by_staff,  Post.joins(comment: user).where(users: {role: 'staff'}).uniq
create_view :uncommented_posts,        "SELECT * FROM posts LEFT OUTER JOIN comments ON comments.post_id = posts.id WHERE comments.id IS NULL"
````
And can be dropped:

```ruby
drop_view :posts_commented_by_staff
drop_view :uncommented_posts, :if_exists => true
```


ActiveRecord works with views the same as with ordinary tables. That is, for the above views you can define

```ruby
class PostCommentedByStaff < ActiveRecord::Base
  table_name = "posts_commented_by_staff"
end

class UncommentedPost < ActiveRecord::Base
end
````

I took this idea from [Schema Plus](https://github.com/SchemaPlus/schema_plus) gem and I opened an early PR to see if this something that Rails Team wants to include, I'll make the change needed and add the proper tests as needed.  For the moment I only tested against to PostgresSQL

Thanks