Feature Request: Recursive copy 

I would like to use this action to copy directory structure, something like:

```
protobuf/
  foo/
    foo1.protobuf
    foo2.protobuf
  bar/
    bar1.protobuf
    bar2.protobuf
```

However, if I set`source_folder: 'protobuf'`, the CI step fail with the following error.

```
cp: omitting directory 'protobuf'
Error: exit with `FAILURE`: 1
```
