This is the source for the chrome extension TextEditAid, which gives you a way to use a GUI editor (or a filter script) on the text in a form's textarea inputs.
You can install it from the chrome extension store, or you can use Chrome's developer mode to load an unpacked extension by browsing to the extension directory.
To use this extension requires that you run some kind of HTTP server, which is typically done on your local host for security reasons. The "servers" directory has a perl script that can be used as a stand-alone server. If you use the extension to edit secret info, be sure that you've reviewed your server setup for safety (particularly the temp-file setup and connection auth). You should also consider cleaning up the temp dir more rapidly (either by setting a 0-hour expiration time in the script or by using other cleaning methods). The perl script defaults to saving your edited text for at least 4 hours (just in case you need to re-grab some recently edited text). However, it only does the expiration step at the end of each HTTP edit request because its logic is concerned about saving space, not about security. Thus, you may want to implement your own server if you need strong security.