Provide fallback for version:bump auto

[michaellihs]

Hi there,

I'd like to suggest a new feature (which does not exist to my knowledge). I'd like to provide a --fallback parameter to the version:bump auto command so that I can set the action to be taken if no flag (#patch #minor #major) is provided in a commit.

Background: I like the idea of auto-bumping but since we are working in a CI setup that triggers further builds on top of a versioning task, the pipeline breaks due to '+build...' version numbers.

If you think this feature is interesting for others, too - I'd be glad to implement it and create a pull request.

Thanks for your work on the tool!

Michael