Add .mm as an XML extension

This pull request adds `.mm` as a file extension for XML as requested in #1690.
This conflicts with Objective-C++. I added a new heuristic rules to solve this.
All XML `.mm` files start with `<map version="[^"]+">`.

I tested  it on the following repositories:
- https://github.com/coolfluid/documents
- https://github.com/ArchieT/ingressmedal
- https://github.com/dokinkon/LecturePlayer
- https://github.com/wybosys/nnt
- https://github.com/roikr/openFrameworks007
- https://github.com/dmentre/re-demexp
- https://github.com/js-scala/samples
- https://github.com/sos-berlin/scheduler-engine-doc

All files were correctly detected.