Add :read_attribute option NOT to override original reader methods.

This change allows you to set `:read_attribute` option in `stores_emoji_characters`.

If you set `read_attribute: false` then:

```rb
class Comment
  stores_emoji_characters :body, read_attribute: false
end

comment.body # => "Hello, :grin: world!"
```

set `read_attribute: {suffix: :unicode}` then: 

```rb
class Comment
  stores_emoji_characters :body, read_attribute: {suffix: :unicode}
end

comment.body         # => "Hello, :grin: world!"
comment.body_unicode # => "Hello, 😁 world!"
```