Cleaned up interfaces a bit.
- Pass an interface{} to formatOutput
- Outputter interface now requires a method for each output type, since each
  output type may require data modification to make it pretty. Example: JSON
  might use the node name as the index, where XML might cater better to anonymous
  members with a name element. This always depends on the CLI functionality, too.
- Don't assume any formatting and don't assume any particular structure in
  formatOutput.
Cleaned up code a bit and added xml tags instead of everything as an element. Maybe this is more correct? XML is confusing.