[Feature Request] Ability to beep on a custom schedule (e.g. 20 minutes)

[dhruuuuuv]

Hi,

Thanks for this great piece of software - it works perfectly and I enjoy using it a lot! A feature I have thought would be nice is the ability to integrate common break-taking practice with the beep feature, such as every 20 minutes (20-20-20 vision rule, or to get up and take a walk). It'd be great to have one less piece of software to do that.

I'm opening this to gauge interest, I'm happy to contribute and open a PR if others think it'd be useful. Thank you!