Masher fails to mash files when moving a directory into src

1. Start `mash --watch`
2. Move a folder full of images into `src/images` (or wherever you store images)
3. Note error:
```
masher error - Invalid file type
       ⨽ ./src/images/test-folder
```

`processPath` should determine if the path is a folder and loop through the contents if so.