Choose installer according to lock file

[rohit-gohri]

Problem

I have both yarn and npm installed on my system as I contribute to various projects, each of which use different tool sets.

This defaults to yarn is yarn is available, which it is on my system, but I use npm for some projects.

Proposed Solution

Detect installer based on lockfile or add a manual override to specify installer

[tabrindle]

Does the --npm option help you?

.option('--npm', 'Use npm to install.')

I think maybe I could make the autodetect a little more elegant. If say a package-lock exists, but no yarn.lock, it should use npm, not yarn as it does now.

[rohit-gohri]

--npm does fix it for me. Didn't see it in the docs.

I can open a PR for this if that'd be okay with you?

[tabrindle]

Of course! PRs are always welcome.