Build version should default to app version on Windows

[kevinsawicki]

Please describe your issue:

On Mac, both plist version keys are set to the app-version option when present, https://github.com/electron-userland/electron-packager/blob/9134f7be4678fdf0144bbb6b00f2da4756566415/mac.js#L109-L111 so you can just set the app-version option when you want the app version and build version to have the same value, but you can still override build-version when needed.

This is not happening on Windows, https://github.com/electron-userland/electron-packager/blob/9134f7be4678fdf0144bbb6b00f2da4756566415/win32.js#L36-L42

Perhaps on Windows, the file-version and product-version should both default to app-version initially but file-version should still be overrideable by build-version when it is present.

This would make the app-version defaulting to the package.json version value introduced in #449 be more consistent on Windows and Mac.

[malept]

I don't have a problem with adding this feature.