##<gold-zip-input>
gold-zip-input is a single-line text field with Material Design styling
for entering a US zip code.
<gold-zip-input></gold-zip-input>It may include an optional label, which by default is "Zip Code".
<gold-zip-input label="Mailing zip code"></gold-zip-input>The input supports both 5 digit zip codes (90210) or the full 9 digit ones, separated by a dash (90210-9999).
The input can be automatically validated as the user is typing by using
the auto-validate and required attributes. For manual validation, the
element also has a validate() method, which returns the validity of the
input as well sets any appropriate error messages and styles.
See Polymer.PaperInputBehavior for more API docs.
See Polymer.PaperInputContainer for a list of custom properties used to
style this element.