Add package-manager option

[hrkt]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• [?] The changes are appropriately documented (if applicable).
• [?] The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

This PR introduces "package-manager" option.
It is a replacement of #614, and it tries to implement the spec defined in #515.

It can parse 'nom', 'yarn' as package-manager option.
When other package manager is specified, a warning message is shown.

This PR does not include the code below.

1. modification of usage.txt
2. unit test

Thanks in advance for reading this message.

Fixes #518.
Addresses #515.

[malept]

In addition to your notes about what's missing:

• it should have cnpm support (which, as I understand it, is the exact same interface as npm)
• there is one more reference to NPM in the tests that needs to be addressed.

[malept]

per the spec:

It should be called packageManager in the API

[malept]

There's a better way to structure this:

opts.packageManager ||= 'npm'

switch (packageManager) {
  case 'npm':
    // do stuff
  case 'yarn':
    // do stuff
  default:
    throw error;
}

[malept]

This should come back in some form or another. We need to be able to see when the prune operation occurs and with what package manager. It's pretty helpful for debugging user problems.

[hrkt]

Thank you, @malept for the review. I'll fix it in this week.

[malept]

For tests, I think what I'd like to do is have Travis/AppVeyor download Yarn/cnpm and make sure they work correctly with pruning.

The test reference to NPM should probably be configurable via an environment variable.

[malept]

I've refactored this PR so that it also works with cnpm, and added docs/tests.

The test reference to NPM should probably be configurable via an environment variable.

I decided against this. I'm OK with having people working on tests needing to have NPM installed.

[malept]

Need to fix calling cnpm on Windows (per CI failure) and also add a NEWS entry.

[malept]

I'm no longer calling cnpm on Windows (see: #515 (comment)). I just need to squash my commits, and I think I'll merge.