Support dropdown list-type custom fields

[fdcds]

When creating custom fields, I have the option to select "dropdown", which results in a .customField[].type="list" being created. The options are predefined in .customField[].options[] with .customField[].options[].value being what github.com/adlio/trello calls cfval. It always contains a text field.

It would be great if dropdowns were supported by this library.