Feature request: Add ability to run custom commands when certain events happen (break starts/finishes/gets skipped/postponed)

Prerequisites

• I'm using latest version: v 1.1.99
• I've checked Known issues
• I've checked Open issues

Description

More of a feature request. I would be interested in running a custom command (for my use case, it is a screen locker) when you are on the long break (and/or when you postpone/skip a break, or when the break finishes?). My use case is so that when a long break starts, I can walk away without worrying that my computer is unlocked.

Operating system: Gentoo

Steps to Reproduce

1. Run Stretchly
2. Wait until a long break starts
3. Walk away from your computer

Expected behavior: My computer will run a custom command when the lock break starts, which then locks my screen.

Actual behavior: No custom command is run, my screen just locks as it normally would without Stretchy (I have a timer, but would like more fine grained integration)

Reproduces how often: 100%

Additional Information

Any additional information, configuration or data that might be necessary to reproduce the issue.

I'd be happy contributing the patch if it is something that would be accepted.