Add password format

Fixes #231

Author: webron

versions/2.0.md

@@ -92,6 +92,7 @@ byte | `string` | `byte` |
 boolean | `boolean` | |
 date | `string` | `date` | As defined by `full-date` - [RFC3339](http://xml2rfc.ietf.org/public/rfc/html/rfc3339.html#anchor14)
 dateTime | `string` | `date-time` | As defined by `date-time` - [RFC3339](http://xml2rfc.ietf.org/public/rfc/html/rfc3339.html#anchor14)
+password | `string` | `password` | Used to hint UIs the input needs to be obscured.
 
 ### Schema