Make `:index` in migrations work with `add_column`

Option `index` works perfect in schema definition constructions like
```ruby
change_table :table do |t|
  t.integer :foo, index: true
  t.references :bar, index: true
end
```

In [documentation](https://github.com/rails/rails/blame/master/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb#L372) I found that `index` should works for `add_column`. I've add tests and functionality for it.