"dotnet package download" should have a switch to download dependencies

### Is your feature request related to a problem? Please describe.

The new `dotnet package download` command is great, but it doesn't quite get rid of our need to use nuget.exe to install packages, because it doesn't bring down dependency packages.

### Describe the solution you'd like

Add a switch `--dependencies` or similar that will download direct and transitive dependencies.
Since following dependencies includes paths that are target framework specific, you may also add a `--target-framework` switch that will allow filtering of those dependencies to a particular target framework.