feat(amTimeAgo): add am-time-ago-formatter attribute

[andreialecu]

Adds optional am-time-ago-formatter which can take a function to alter the displayed text
based on custom logic. Example:

<span am-time-ago="message.time" am-time-ago-formatter="customFormatter(message.time)"></span>

$scope.customFormatter = function(date) {
   // any date within 5 minutes from now or in the past will be displayed
   // as 'in a few moments'
   if (date > Date.now() - 5 * 60000) {
      return 'in a few moments';
   }
};

Fixes:
#44
#153

[angelxmoreno]

we ever going to merge this?

[alex88]

this would be great!

[shirazSalpo]

How can i get the angular-moment version with m-time-ago-formatte