Add check if there is text 'off by default' in index.

See issue #865.

I ran `node Makefile.js checkRuleFiles` and checked manually the result.
It works fine.

If you agree, I would add in this PR the fixes for the index.

```
Validating rules                                                                                       
Missing '(off by default)' for rule block-scoped-var in index                                          
Missing '(off by default)' for rule brace-style in index                                               
Missing '(off by default)' for rule complexity in index                                                
Missing '(off by default)' for rule default-case in index                                              
Missing '(off by default)' for rule func-names in index                                                
Missing '(off by default)' for rule func-style in index                                                
Missing '(off by default)' for rule handle-callback-err in index                                       
Missing '(off by default)' for rule max-nested-callbacks in index                                      
Missing '(off by default)' for rule no-div-regex in index                                              
Missing '(off by default)' for rule no-else-return in index                                            
Missing '(off by default)' for rule no-eq-null in index                                                
Missing '(off by default)' for rule no-extra-parens in index                                           
Missing '(off by default)' for rule no-floating-decimal in index                                       
Missing '(off by default)' for rule no-lonely-if in index                                              
Missing '(off by default)' for rule no-mixed-requires in index                                         
Missing '(off by default)' for rule no-nested-ternary in index                                         
Missing '(off by default)' for rule no-new-require in index                                            
Missing '(off by default)' for rule no-path-concat in index                                            
Missing '(off by default)' for rule no-restricted-modules in index                                     
Missing '(off by default)' for rule no-self-compare in index                                           
Missing '(off by default)' for rule no-ternary in index                                                
Missing '(off by default)' for rule no-warning-comments in index                                       
Missing '(off by default)' for rule one-var in index                                                   
Missing '(off by default)' for rule quote-props in index                                               
Missing '(off by default)' for rule radix in index                                                     
Missing '(off by default)' for rule sort-vars in index                                                 
Missing '(off by default)' for rule space-after-keywords in index                                      
Missing '(off by default)' for rule space-in-brackets in index                                         
Missing '(off by default)' for rule space-unary-word-ops in index                                      
Missing '(off by default)' for rule wrap-iife in index                                                 
Missing '(off by default)' for rule wrap-regex in index                                                
```