Add optional libclang implementation

[Notgnoshi]

includegraph.py is trivial to implement, if you give up some requirements:

• standard library only dependencies
• detect circular dependencies
  • related: generate the graph that the source code defines, not the graph that was actually followed by the preprocessor (because of header guards)

[Notgnoshi]

Or perhaps just use cpp -H?

$ cpp -H -I/home/nots/Documents/includegraph/examples/example3/include -I/home/nots/Documents/includegraph/examples/example3/src /home/nots/Documents/includegraph/examples/example3/src/example3.cpp -o /dev/null
. /home/nots/Documents/includegraph/examples/example3/include/example3/foo.h
. /home/nots/Documents/includegraph/examples/example3/include/example3/bar.h
. /home/nots/Documents/includegraph/examples/example3/src/private.h
.. /home/nots/Documents/includegraph/examples/example3/src/circular.h
... /home/nots/Documents/includegraph/examples/example3/src/private.h

... this can detect circular includes, so there's no actual reason to parse the linemarkers...