Add reference to ValueTuple package to NetStandard2.0 meta-package

[jcouv]

Although the ValueTuple and related types won't be part of the 2.0 standard, the meta-package which supports the experience in VisualStudio will reference some additional types for convenience.

@weshaggard @ericstj @tarekgh @terrajobst for information and triage.

[jcouv]

Assigned to Wes for triage

[weshaggard]

@jcouv this is on the radar and I will use this issue to track including it once we get the packaging all setup.

[jcouv]

@weshaggard Just a friendly ping on this. I don't remember which release this is supposed to be included in and what deadlines apply.

[weshaggard]

This will be part of .NET Standard 2.0 which will be released with .NET Core 2.0 time frame. It will still get done.

[weshaggard]

Another thing to point out is that we will need to wait to do this until we can update the ValueType package itself to build against netstandard2.0.

cc @ericstj

[terrajobst]

Same answer as for #220: we'll put in netstandard.dll and put the implementation for .NET Framework into a new package that provides support for .NET Standard on .NET Framework 4.6.1.