Rejig formatting API. closes flarum/core#85

It works but it’s not the most pretty thing in the world. @franzliedke
Would be great if you could take a look at the whole formatting API and
work your magic on it sometime… my brain is fried!

js/lib/models/post.js

@@ -12,7 +12,7 @@ Post.prototype.user = Model.one('user');
 Post.prototype.contentType = Model.prop('contentType');
 Post.prototype.content = Model.prop('content');
 Post.prototype.contentHtml = Model.prop('contentHtml');
-Post.prototype.excerpt = Model.prop('excerpt');
+Post.prototype.contentPlain = computed('contentHtml', contentHtml => $('<div/>').html(contentHtml.replace(/(<\/p>|<br>)/g, '$1 ')).text());
 
 Post.prototype.editTime = Model.prop('editTime', Model.date);
 Post.prototype.editUser = Model.one('editUser');