paket parser should handle paket groups

[haf]

See haf/expecto#4 (comment) for discussion.

Sample file:

source https://dotnet.myget.org/F/dotnet-core
 source https://www.myget.org/F/fsharp-daily
 source https://www.myget.org/F/netcorecli-fsc-preview3

 github logary/logary src/Logary.Facade/Facade.fs 4b21d888e83d72d1238ab0f36110e2a3946240e7

 nuget Argu
 nuget FsCheck
 nuget FSharp.Core ~> 3
 nuget FSharpx.Core
 nuget BenchmarkDotNet
 nuget NuGet.CommandLine

 group build
     source https://www.nuget.org/api/v2
     nuget FAKE

 group netcore
     source https://www.nuget.org/api/v2

Required 'context' is what group the paket.references file uses for the given referenced nuget.