WIP: Create separate commits per upgraded package

[olpeh]

Based on my suggestion in #37, here is a PR that implements that functionality.

This is still a bit of work in progress, but I wanted to open this PR in order to get feedback. I used this solution now for my project and it seems to be working well for my use case.

Here's the list of things to do:

• [ ] Introduce an option in .npm-upgrade.json which will store the name of the package manager used in current project. This option is not mandatory and may not be specified.
• [ ] If it present, use it. If it's not, goto 3.
• [x] Search for yarn.lock or package-lock.json files. If they present, use corresponding package manager (without setting an option in config file).
• [ ] If lock files are not there, ask user in commit phase about package manager and set it in the config.

Implements and closes #37

[olpeh]

Good feedback, I'll see when I have time to continue working on this.