Support subdirectories in bucket object name 

It would be nice to somehow indicate in which subdirectory the source zip file should be placed.

For example, I have a single bucket for all cloud functions sources, and I want to keep a file layout like this:

```
my-bucket/
    function1/hash-build.zip
    function2/hash-build.zip
```

My suggestion is to add an optional variable for object directory path.