Add callback to be triggered right before transition

[karkipy]

Fixes #34

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if necessary)
• I have performed adequate tests that prove my fix is effective or that my feature works

Proposed Changes