In case if we want to add a column into the existing table
with index on it, we have to add column and index in two
seperate lines.
With this feature we don't need to write an extra line to
add index for column. We can just use `index` option.

Old behaviour in action:
```
  change_table(:languages) do |t|
    t.string :country_code
    t.index: :country_code
  end
```

New behaviour in action:
```
  change_table(:languages) do |t|
    t.string :country_code, index: true
  end
```

Exactly same behaviour is already exist for `create_table` migrations.