Add option to override commit message

Hey,
Thanks for the gem :+1: I have a small addition:
We use the gem on our CI to automatically bump version every time master build passes. Which worked perfectly until we tried to push commit with changes to VERSION file. That commit triggered another build and another bump and so on forever .. :)
So we just want to customise commit message to add a special flag for CI to ignore those commits.

Not sure do you want me to try to add tests for this case? I can't find any tests for this part of code