Add example that forwards calls to the real filesystem

[vi]

libfuse has fusexmp.c, so fuse-rs should be examples/xmp.rs.

Supposed to just mirror root filesystem using libstd's std::fs::* operations.

The example is rather crude and not all operations are supported, but a simple cargo init ; cargo run test works on that filesystem.

P.S. Only now stumbled upon https://github.com/wfraser/fuse-mt/tree/master/example.