Kson new functions for writing to build object manually and write to string.

[BeppeInfo]

I'm trying to expand Kson JSON parsing library to something more complete, allowing for writing to a string as well (and making the kson_format function use it). As the previous kson_t structure (putting all the nodes on a flat list), although being more easy for cleaning up, makes it more difficult to add new nodes, I changed it a bit, making it more like a tree of kson_node_t's, modifying the parsing accordingly.

I hope for some feedback it. Thanks.

[BeppeInfo]

I did this as it was needed for my project, but do you think it is useful for merging, @attractivechaos , @lh3 ?