Support netstandard < 1.6 ?

[kal]

Currently all the projects are targeting netstandard 1.6 as the "latest and greatest" .NET Core. This does restrict the cross-platform compatibility a bit for older platforms such as UWP, Windows 8/8.1 and Windows Phone.

It would be nice to be able to also target at least netstandard 1.4 (for UWP 10.0 compatibility), 1.2 (Windows 8.1, WP 8.1) and 1.1 (Windows 8.0) and possibly also 1.0 (WP Silverlight 8.0).

NOTE: As Xamarin is supposed to be keeping up now with netstandard releases, NOT doing this stuff should not impact cross-platform support on the Xamarin target platforms (macOS, Linux, iOS or Android).