Add support for delta (multi value fields)

As there is mentioned in getFieldValue() function:

// @TODO: We should add support for deltas.

Currently the module lacks support for multivalue fields (speaking for 8.x version). Moreover it even breaks your site if you use a multivalued field and set the linked field formatter.

Therefore I will try to provide support for multivalue fields. I already have a basic local version with multivalue field support. I will improve this and test and then provide patch here later. So I am assigning this task to myself now.