Add default format (angularMomentConfig.format config property)

Added angularMomentConfig.format for a "global" default value to be used for all dates. This can be overridden by setting the am-format attribute.

__Use case:__
All my dates are in UTC, but I want to show the user relative local time. The current method is to use the am-format property `<span am-time-ago="bid.time" am-format="YYYY-MM-DD HH:mm:ss Z"></span>`. With many dates this was clutter, so I prefer to define a default format: `app.constant('angularMomentConfig', {format: 'YYYY-MM-DD HH:mm:ss Z'});` instead.