Make Discourse.ShareLink easy for plugins

For example, this javascript code, if added by a plugin, would enable a Tumblr share link:

Discourse.ShareLink.addTarget('tumblr', {
  iconClass: 'fa-tumblr',
  generateUrl: function(link, title) {
    return ("http://www.tumblr.com/share/link?url=" + encodeURIComponent(link) + "&description=" + encodeURIComponent(title));
  },
  shouldOpenInPopup: true
});
Merge pull request #2122 from riking/extensible-share

Make Discourse.ShareLink fully extensible