Add initial support for custom date formatting, refs T223

Summary:
- add `Ushahidi\Tool\Date` class
- add "date_format" key to site config
- include spec test

Test Plan:
```
bin/phpspec run spec/Ushahidi/Tool/DateSpec.php
```

Reviewers: vladimir, aMoniker

Reviewed By: vladimir

Subscribers: srutto, rjmackay, sheesh, bodacea

Maniphest Tasks: T223

Differential Revision: https://phabricator.ushahidi.com/D466