Allow Shell provisioner to redirect its output into a logfile

I am using a shell provisioner which writes a lot of output to stdout.
Most of the times, this output just spams my build log and I do not need, but occasionally I want to have a look at it for debugging purposes.

What I would like, is to have an extra option for the [shell-provisioner](https://www.packer.io/docs/provisioners/shell) to specify a local file (on the build host) where all output from my script gets redirected to.