Pugixml is now downloaded if not found, and the CMake messages are now
clearer that the dependency is downloaded from the internet.
The POSIX getline function is now replaced with std::getline, which
works on MSVC.
Now you can easily parse EXL files, and this commit includes changes
to GameData to easily extract a EXH from just an excel sheet name
like "Item" or "Map". However this still doesn't work for language
seperated excel sheets yet.
