Add rule "Age" to validation.

Add "Age" rule in favor of "MinimumAge".

Age rule works pretty much the same way “minimumAge”
worked but it optionally accepts a maximum age as second
argument.

### Blockers for merge

- [x] Add `age` validator on README.
- [x] Modify README to reflect removal of `minimulAge` (point deprecation in favor of `age`).