Addressing issue where npm isn't found on windows

[gbivins]

This change adds 2 things

1. Method to get a name for the npm command based on platform (windows vs others)
2. Use the method to get the platform specific name for npm so that setup tools succeed

I tested on my generated widget code but I didn't see any other way to test this change.
If there is, I'd be more than happy to run them to verify the changes.

Fixes #20

[jasongrout]

Thanks!

[gbivins]

@jasongrout Just saw this! My first accepted PR on github outside of my own repos 😄

[jasongrout]

Congratulations!