Add regex format for US ZIP codes

This one's [pretty well-documented](http://en.wikipedia.org/wiki/ZIP_code).  US ZIP codes consist of 5 digits.  Optionally, 4 additional digits can be affixed to the 5 primary digits (separated by a hyphen).  Believe checking that a zip code exists is beyond the scope of this configuration.

__Proposed pattern__
`^\d{5}(?:[-\s]\d{4})?$`

__Examples__
- 98103
- 90210-9998