Make paket add accept multiple NuGet packages #2756

In some rare cases (Forge scaffolding, for example) I'd want to add multiple packages at the same time. At the moment, we add packages one-by-one in the loop, what means that the whole process (starting Paket, resolver, I/O) is run multiple times... which adds a lot of overhead and is pretty long operation. It'd be really helpful to allow us add those packages at once.

Alternative solution would be adding flag to paket add that would disable resolver - in such case command would just add entry in paket.dependency without changing paket.lock and project files.