Use `--output` option instead of stdout redirect

[kasraghu]

On windows the redirect operator > creates an utf-16 encoded ignition file. This change avoids the issue https://github.com/coreos/butane/issues/293.

[jlebon]

Thanks for the patch! On Linux at least, a nice advantage of using redirection is that it works better with the container flow. E.g. if a user defines that butane alias higher up to run it as a container instead and substitute example.ign to some path outside the working directory, --output won't work as intended, but redirection would. WDYT about instead adding a NOTE in the Windows section above that recommends using --output on that platform?