No concept of 'upgrades'

[ethanbergstrom]

Since PackageManagement and corresponding DSC resource don't natively support the concept of an 'Upgrade-Package' cmdlet, we could expose the concept of an upgrade with the use of a special keyword passed to Get-Package's RequiredVersion parameter that triggers Find-Package's logic to check for the latest version available before checking to see if that version is already installed. The DSC LCM could, in effect, keep the package continually up to date.

# CLI example
Get-Package python -ProviderName chocolateyget -RequiredVersion latest

# DSC Example
PackageManagement PythonChoco {
	Name = 'python'
	RequiredVersion = 'latest'
	ProviderName = 'chocolateyget'
}

Probably could only work if one (and only one) source is available to the provider / configured in chocolatey (Chocolatey.org by default). Example enhancement based on my multi-source branch: ethanbergstrom/ChocolateyGet@399bfa0 with help from @matthewprenger

Will hold on PR until after the multi-source PR is resolved. Fixed in #15