Allows defining array variables in an inventory file

Currently we can define only simple string variables in an inventory file, but I think it would be nice if we could put array variables as well.

```ini
[cluster]
node1
node2

[cluster:vars]
log_path = /log
data_paths[] = /data1
data_paths[] = /data2
data_paths[] = /data3
```

I know this can be easily done with YML vars file, but when I want to run the same playbook, and its corresponding vars files, on a different set of machines with slightly different configurations, I find it more convenient to put cluster-wide variables in each inventory file.

Let me know what you think. Thanks.