Request: Add support for downloading preview modules

[brajjan]

Currently not supported to download preview modules from powershell gallery as there is no AllowPrerelease parameter

Install-Module : The '-AllowPrerelease' parameter must be specified when using the Prerelease string in MinimumVersion, MaximumVersion, or RequiredVersion

All preview powershell modules version name are named preview (eg. Name = ComputerManagementDsc, Version = 8.5.0-preview0001)