code: function tag2(name, tmpl, css, attrs, fn) {
        if (css) { styleManager.add(css, name); }
  
        __TAG_IMPL[name] = { name: name, tmpl: tmpl, attrs: attrs, fn: fn };
  
        return name
      }
docs: Create a new riot tag implementation (for use by the compiler)
  @param   { String }   name - name/id of the new riot tag
  @param   { String }   tmpl - tag template
  @param   { String }   css - custom tag css
  @param   { String }   attrs - root tag attributes
  @param   { Function } fn - user function
  @returns { String } name/id of the tag just created