Allow modifying an existing instance of VS

[kzu]

The vs install command currently assumes you're installing from scratch.
Proposed syntax for modifying:

vs modify [nickname] [options]

Options would be mostly the same as for install.

Examples of the proposed syntax by @adalon are:

vs modify master ++xamarin --uwp  // add Xamarin workload and remove UWP
vs modify master ++workload1 ++workload2 
vs modify master --workload1 --workload2 .

We'd need to modify the way we parse the workload ID aliases for install, which currently render
as follows with vs install -? (but do allow passing in a + instead of --):

      Workload ID aliases:
      --mobile               --add Microsoft.VisualStudio.Workload.NetCrossPlat
      --xamarin              --add Microsoft.VisualStudio.Workload.NetCrossPlat
      --core                 --add Microsoft.VisualStudio.Workload.NetCoreTools
      --azure                --add Microsoft.VisualStudio.Workload.Azure
      --data                 --add Microsoft.VisualStudio.Workload.Data
      --desktop              --add Microsoft.VisualStudio.Workload.ManagedDesktop
      --unity                --add Microsoft.VisualStudio.Workload.ManagedGame
      --native               --add Microsoft.VisualStudio.Workload.NativeDesktop
      --web                  --add Microsoft.VisualStudio.Workload.NetWeb
      --node                 --add Microsoft.VisualStudio.Workload.Node
      --office               --add Microsoft.VisualStudio.Workload.Office
      --python               --add Microsoft.VisualStudio.Workload.Python
      --uwp                  --add Microsoft.VisualStudio.Workload.Universal
      --vsx                  --add Microsoft.VisualStudio.Workload.VisualStudioExtension

In typical command line parsing, --[flag is treated as a boolean option you turn on with the --.
However, the proposed syntax intends to make the -- mean "remove" and an alternative ++ mean
"turn on/enable/add". Maybe we can use a single dash/plus to tell apart both from the regular non-short
form of a "standard" argument name instead?

[kzu]

Done by @adalon