Make `out` on `native_binary` optional

[tpudlik]

Fixes: #399

[tpudlik]

We could avoid the .exe suffix on non-Windows machines by adding a is_windows attribute populated from a select in a wrapper macro (like in https://github.com/bazelbuild/bazel-skylib/blob/main/rules/private/copy_file_private.bzl#L140-L143), but adding a wrapper macro for such a minor aesthetic benefit seems a little overkill. Let me know if you'd like me to do it, though!