New option: bin link type

This is an implementation of https://github.com/npm/npm/issues/6758. It adds a `bin-link-type` option:

### bin-link-type

* Default: `auto`
* Type: String

Tells npm what kind of bin links should be created. It defaults to `auto`, which means it is based on the environment. If set to `win`, `.cmd` shims will be created. If it's `unix`, only symlinks will be used. If it's `both`, well, both will be created. This is useful when you have a VM setup with a Windows host and a Unix guest, and want to run `npm install` on your guest and still have the `.cmd` shims available on your Windows host.