grammar: try to detect regex values to avoid weird highlighting

grammars/fluentd.cson

@@ -138,6 +138,10 @@ repository:
           {include: 'source.json'}
         ]
       }
+      {
+        match: '\/.+\/$'
+        name: 'value.string.regex.fluentd'
+      }
       {
         match: '\\b(\\d+)\\b'
         name: 'value.constant.integer.fluentd'