Customising "Add key" and "Add item" texts

[ErikKoinberg]

Building on issue #149, it would be nice to be able to change the "Add item" text for adding new items to an array. Either fully customisable or using some kind of Item type name. Changing it to "Add book" or "New book" would be cool.

[bhch]

This is being discussed on the JS repo: https://github.com/bhch/react-json-form/issues/89.