remove unused variable
user-defined check_attributes methods

this lets you define your own attribute checkers, e.g.

 parseRules: {
   tags: {
     div: {
       check_attributes: {id: function(){ ... }}
     }
   }
 }
support arbitrary check_attributes via wildcard

for example, you could whitelist all data- attributes on divs like so:

 parseRules: {
   tags: {
     div: {
       check_attributes: {'data-*': function(s){ return s; }}
     }
   }
 }