Added exact as valid modifier (#269)

Author: mokkabonna

lib/rules/valid-v-on.js

@@ -18,7 +18,7 @@ const utils = require('../utils')
 const VALID_MODIFIERS = new Set([
   'stop', 'prevent', 'capture', 'self', 'ctrl', 'shift', 'alt', 'meta',
   'native', 'once', 'left', 'right', 'middle', 'passive', 'esc', 'tab',
-  'enter', 'space', 'up', 'left', 'right', 'down', 'delete'
+  'enter', 'space', 'up', 'left', 'right', 'down', 'delete', 'exact'
 ])
 const VERB_MODIFIERS = new Set([
   'stop', 'prevent'