Support SerializationFeature.WRITE_EMPTY_JSON_ARRAYS

[cowtowncoder]

(note: cleaved from #7)

It would be nice to support this SerializationFeature, along with other options to filter out "empty" items of various types.

[cowtowncoder]

Hmmh. Looking at code in jackson-databind, this may be surprisingly difficult to make work as expected -- problem being that check is based on JavaType being container (array, Collection, Map), which is something that ArrayNode and ObjectNode are not.
May need to see if we could change handling in 2.7 or later.