Add support for PostgreSQL `interval` data type with conversion to
`ActiveSupport::Duration` when loading records from database and
serialization to ISO 8601 formatted duration string on save.
Add support to define a column in migrations and get it in a schema dump.
Optional column precision is supported.

To use this in 6.1, you need to place the next string to your model file:

    attribute :duration, :interval

To keep old behavior until 6.2 is released:

    attribute :duration, :string