Support for nested documents within custom formatting

Hello,

Just took a crack at a pretty major enhancement here... Being able to specify fields is great, but it would be even better if we could specify the document structure in the NLog.config file. For example, I could define my own structure for an Exception document to leverage custom field formatting, without giving up the Document object that it's stored in in the default case. 

Let me know if there's anything I could have done better, or if you had another thing in mind for how this might be implemented. Thanks!