Add project_stats script

Queries git database to get statistics on project or release.
Usage in `project_stats -help`

Example:

```bash
$ project_stats v0.20.0
Unique contributors since v0.20.0
  Casks	     173
  code	      19
  docs	      16
  any	     181

All-time contibutors	     450

Casks created since v0.20.0	     292
Total current Casks in HEAD	    1028
```