Fix parameters when using reset

Previously, `true` was passed when in `reset: true` mode and a rule
was turned on by passing `true` as well.  Now, `undefined` is passed
instead.

lib/index.js

@@ -121,7 +121,7 @@ function attachFactory(id, rule, options) {
             rule(ast, file, options, done);
         }
 
-        return options === false ? null : plugin;
+        return plugin;
     }
 
     return attach;
@@ -267,7 +267,13 @@ function lint(remark, options) {
         known.push(id);
 
         if (!(setting === null || setting === undefined)) {
+            /* Pass turned on rules `undefined`. */
+            if (reset && setting === true) {
+                setting = undefined;
+            }
+
             if (setting === false) {
+                setting = undefined;
                 disable.push(id);
             } else {
                 enable.push(id);