add an option in the config to allow users to create their own window title mappings that postpone a break

break-time has a built-in plugin to look for X windows with a given title. If it finds these windows, it postpones an upcoming break.

This is used to make sure that break-time doesn't pop up when you're in a video chat.

All of the code for this is in this module:

https://github.com/cdepillabout/break-time/blob/6bad134cf96089ebd0ee372412d83d896b9f8633/src/scheduler/plugins/window_titles.rs

Right now there is only support for using [Google Meet](https://meet.google.com/). This should be extended to account for other video chat sites/programs, like Skype, Zoom, etc.

Also, there should be an option added to the user's config to allow the user to add support for different X programs.

Here's the module that controls config options:

https://github.com/cdepillabout/break-time/blob/6bad134cf96089ebd0ee372412d83d896b9f8633/src/config.rs