Adding support for fill_existing_with on add_belongs_to

[jwoertink]

Fixes #17
Fixes #22

Let me know if this needs to do more.

[paulcsmith]

I think this looks great! I think eventually what would be awesome is allowing users to pass a block to add and add_belongs_to when in an alter statement. The block would basically run with no args which would allow you to set up all the data before it actually makes the column required:

alter table_for(User) do
  add otp_code : String, fill_existing_with: ->{
    UserQuery.new.each do |user|
      SaveUser.update!(user, otp_code: generate_code_for_user(user))
    end
  }
end

So it'd add the column as allow null, then run the block, then make it required. Would be mega helpful I think! But that can be done some other time IMO