ipc: allow sending commands with named arguments

This has been part of the libmpv for a while, so the implementation in
the IPC code is quite simple: just pass the mpv_node representing the
value of the "command" field without further checks to
mpv_command_node().

The only problem are the IPC-specific commands, which essentially have
their own dispatch mechanism. They expect an array. I'm not going to
rewrite the dispatch mechanism, so these still work only with an array.
I decided make the other case explicit with cmd==NULL. (I could also
have set cmd=="", which would have avoided changing each if condition
since "" matches no existing command, but that felt dirty.)