Adding postal code format for Taiwan

Very straightforward. Taiwanese postal codes consist of 5 digits only; typically only the first three digits are required. The last two digits refer to a more detailed level of locale and are typically optional.  [Data from Google](http://i18napis.appspot.com/address/data/TW). 

__Proposed format__
`^\d{3}(\d{2})?$`

__Examples__
- 106
- 10603