feat: add CLI command

[jedwards1211]

Thanks for this great package!

I added a simple CLI command, you just run merge-package.json in your project directory
after a merge failed with merge conflicts in package.json, and it writes
the merged output to package.json. It uses git show HEAD:package.json,
git show MERGE_HEAD:package.json, and also git show with the output of
git merge-base HEAD MERGE_HEAD to get the input for your existing Node API.

[kellyselden]

This looks pretty cool! Thanks! Can you add a test please to prove that it is working?

[jedwards1211]

@kellyselden done, and I used some real-world package.jsons in a fixture :) the tests use sync fs and child_process operations. I can make them async if you want, but if so I'll need to add fs-extra and promisify-child-process as dev dependencies.

[kellyselden]

I want to give this a proper review soon. I noticed your fixtures have a lot of code unique to your project. Could you perhaps limit the code samples to the bare minimum? GitHub search is bad and I would like these to not show up in my day to day code searches.

[jedwards1211]

Oh I see. By code samples you mean package.json files right? Yeah I could whittle them down