Make the isNpm() check optional with a flag

[alexccl]

I am currently working on a project that would still like the notification output when running in a npm script context. Currently we have to make assumptions about how both update-notifier and isNpm are implemented and manually manipulate process.env to work around this issue. This is not ideal.

I wanted to open a PR to add an notifyWithNpm flag to the notify options. Only if explicitly set would the notification still be outputted in an npm context (not break current functionality). Would this be something you all would be open to?