Export it stationary to json

Enable items of type `it_stationary` to be loaded from json.

Before merging this we should speak about the names that got used here:

`it_stationary` (C++ class name) - it's a flyer (item not furniture). This should be changed, as should `STATIONARY_ITEM` (type of the json object, like `GENERIC`). Maybe something like `NOTE`, `RANDOM_TEXT_ITEM`, or something?

I extended the snippet loading to allow arrays instead of having each snippet in a separate entry. This can be used to create the newspaper item from the forum: http://smf.cataclysmdda.com/index.php?topic=6218.0