Option to pass OS override for Odd WSL configurations

[natefabian18]

I run WSL at work with a program to use my window machine as an Xorg server. I would expect that since im running under the Xorg server that it would send a Linux notification but because of the OS check in the index.js checking for WSL. Added by this pull request it seems (https://github.com/mikaelbr/node-notifier/pull/260). It defaults to sending the windows notification. I agree for most users case this is the correct action to send a notification to the parent window OS but it would be greatly appreciated if there was an option I could pass in to override this default behavior and have it send to the linux OS. Thank you for your time.