Add support for custom field types

[Programie]

Jira plugins are able to add new field types in which case this library can't be used to manage issues with those custom field types.

Would it be possible to allow to extend the existing field types (e.g. by specifying a callback which is then called before doing the mapping in Field:: getSaveValue())? Simply extending the Field class does not work for me as this also means I have to modify any class creating new instances of the Field class to use my own Field class.