Add json-file option to choose a file to be merged with package.json

[desjoerd]

I needed the option to choose a file instead of using "cat" to include the package.json to be merged. This is because I need to use CMD or Powershell which doesn't have "cat" with the same escaping as bash.

I hope that this change will be merged.

[jackboberg]

This seems sensible, I'll test locally...

[theworkflow]

@desjoerd thanks for the PR! Can you add some tests?

[jackboberg]

@harlanj there is currently no test coverage for bin/demeteorizer. Seems like a big request create for a small addition...

That said, if you want to do the legwork 💯

[desjoerd]

Sorry about the late comment! I will take a look how I can add some tests. I see two options:

1. add a unit test file for bin/demeteorizer
2. move the json parsing to lib/update-package, but then it won't warn as fast about a faulty json.

I think option 1 is the best option. What do you guys think?

[theworkflow]

@desjoerd sorry for being so late on this. Can you fix the merge conflict so we can look this over again?