Using log interpolation function
Created a windows-shell provisioner to navigate around unix expectations in upstream Packer provisioner.

Upstream provisioner will always chmod a file and put a shebang of `#!` for inline functions, neither of which work on native Windows envs.

The `windows-shell` provisioner will set reasonable defaults to save repetition in packer configs and works nicely on inline and file scripts.

NOTE: There is no initial support for env vars but that is forthcoming.

Strategy may change if/when https://github.com/mitchellh/packer/pull/1245 gets merged into Packer.