Itsme adding addgroup command

[nlitsme]

I added two commands to mdtest: addgroup and jsonadd, both create a new whatsapp group, add members, and set the topic. jsonadd reads the group's spec from a .json file, addgroup takes it from the commandline.

I extended CreateGroup to report which participants need to be invited, so the addgroup command can send invites those user after creating the group.

Also I fixed a bug in the binary encoding of Nodes with an empty attribute value.

[nlitsme]

I resolved the merge conflicts. and added a 'help' command.

[tulir]

I think I added the group invite error handling at some point already.

mdtest is really only meant for testing things while developing whatsmeow rather than a fully featured CLI client, so probably not going to add more commands there