paket add should sort packages by name

Like issue #187 paket add should sort dependencies while adding (maybe with an additional flag/option)

Since this file is often maintained by hand I don't think that should be default behavior (at least not reordering existing entries).

I think we could try to preserve alphabetical order. Paket add could try to assume alphabetical order and add whereever it fits best. we should probably not reorder the whole file.