Adding pnpm support

[zkochan]

Adding pnpm support should be really easy.

I see currently this tool checks for a lockfile to decide what pm should be used for installation. pnpm has its own lockfile called shrinkwrap.yaml.

Alternatively, which-pm can be used, which will correctly identify the pm even when installing was done w/o a lockfile creation

[frontsideair]

Both ideas sound great, but let's handle them separately. Would you like to implement pnpm support? I can do that if you'd not prefer to. A separate issue would be great for adding which-pm dependency, which I have briefly checked yet, but looks solid.

[zkochan]

great, I'll create a PR