Add capabilities support to feedItem

[kbromer]

Support an optional capabilities argument to allow the user to pass JSON-structured Chatter capabilities to be appended to the feedItem such as 'link', etc.

Capabilities

var id = '00580000005eABFAA2';
var text = 'Post text';
var capabilities = {'link': { "urlName":"My Link","url":"http://www.mylink.com"}};
org.chatter.postFeedItem({id: id, text: text, capabilities: capabilities}, function(err, resp){});