Search and Replace in Multiple Files

Unfortunately I have a client who insists on using 1990's technology for their website and does not want to use a CMS.  This makes tiny little changes to the site take a very long time because every single html file has to be visited and changed.  When I went online to search for an existing search and replace program that would work on multiple files I came across a bunch of programs I either had to pay for or were too shady, so I decided to code up my own app real quick.  The resulting app is attached (source code too).