proj stats: modified Cask count, autofind release

- per conversation in IRC, generate Casks-modified-since-last-release
- render Cask stats in same format used in CHANGELOG.md for 0.27.0
- add 'release' alias to automatically calculate since the most recent tag:

```shell
$ project_stats release
Unique contributors since v0.27.0
  Casks	       7
  code	       2
  docs	       1
  any	       8

All-time contributors	     461

2 Casks added (3 updated) by 7 contributors since v0.27.0
Total current Casks in HEAD	    1052
```