Split out `app.conflicts` usage

[doowb]

As mentioned in this updater-contributing issue, I'd like to use generate-contributing to be able to generate new contributing files if they don't exist when updating projects.

generate-contributing currently expects app.conflicts to exist which relies on other generate features that update doesn't have. I'm proposing that we split out the app.conflicts part and use a library that can be directly used in generate-contributing. This will allow us to use generate-contributing as an updater in updater-contributing.

[jonschlinkert]

I'm proposing that we split out the app.conflicts part and use a library that can be directly used in generate-contributing

It seems like we should add the conflicts plugin to update. thoughts?

[doowb]

I think that's a good idea.

I just realized this morning that updater-gitignore will show a conflicts message when updating the .gitignore file. Maybe I was configuring something wrong and it the conflicts plugin is already available. If that's the case, then we wouldn't need to change anything.