scte35: support decoding splice_schedule command type

[ollytom]

We support encoding splice_schedule commands (see packEvents), but we don't support decoding them yet. For consistency, we could have two functions, unpackEvent and unpackEvents in command.go after packEvent.

We'll need accompanying tests in command_test.go. For example, a test could encode then decode the same splice_schedule command then test equality of the returned []Event.