fixes #271

* feat: Add `Builder::permissions()` method.

With it it's possible to change the default mode of tempfiles on
unix systems.

The example also doesn't hide the fact that it is currently only
useful on Unix, but as the ecosystem matures, thanks to the usage
of `std::fs::Permissions` it should be able grow into more platforms
over time.

* Use `permissions` field of `Builder` on Unix to affect file mode

The implementation favors the least invasive solution even though
its forced to pull platform dependent code up.

The alternative would have been to alter the method signatures
of `create_named()` on all platforms.

* Respect module boundaries and make all platforms permissions aware.

This way, permissions can be passed and handled by platform specific
code, which avoid having to use platform dependent code in the top-level.

* Fail on Windows if somebody tries to set permissions there.

That way, there will be no surprises as would be the case with
doing nothing.

* Add support for setting permissions on directories as well.