Add optional way to track positions in parser

[bwrrp]

For some applications it would be helpful to have a way to map a given node back to its corresponding span(s) in the source XML. We could add an optional map argument to the parse options that can be populated with a mapping of nodes to such position information.

TODO:

• extend internal parse events with position information where necessary
• decide which spans to expose for which nodes:
  • text nodes may be actual character data + references, should these be a single span or multiple?
  • elements have a start and end - single span including contents or separate spans for start and end tags
  • what about attributes?
  • other bits of data? names?
• add option
• convenience methods to translate start and end offsets to column + line?