Implement optional line information

[zeux]

This issue was filed on Google Code: https://code.google.com/p/pugixml/issues/detail?id=79

Sometimes the current offset information is hard to use and users are willing to sacrifice some additional performance/memory to translate offsets into lines.

Right now it can be done externally (http:https://stackoverflow.com/questions/21003471/convert-pugixmls-result-offset-to-column-line/21014028#21014028) but this does not work if encoding conversion has to be performed.