Add support for stopping the ALEFix pipeline early

It would be nice to be able to write a function for use in `ale_fixers` which can return a special value to prevent further tools in the pipeline from being run. This would be useful for fixing some code slightly which requires some human intervention, before being able to run the of the tools. A good example is automatically merging some parts of code with git merge conflicts, and leaving some other conflicts for humans to fix.