Implement `add_conflicts`. Solve #2383.
Implement `add_conflict`. Usage is similar to `add_dependency`.
Merge branch 'add_conflicts' of https://github.com/take-cheeze/mruby into take-cheeze-add_conflicts
remove add_conflicts now; use add_conflict; ref #2390
git push origin masterMerge branch 'take-cheeze-add_conflicts'