Allow additional waveform generator kwargs

We've had requests from a few places to allow additional kwargs to be passed through to the waveform generator. I think this is something we should aim to include, probably after our next release.

Steps that I think are needed to take to support allowing additional waveform kwargs

• add to the parser near here, probably we want to add this as a dictionary as we do with other arguments
• make sure the kwargs are extracted in the required places, e.g., here, here
• add the kwargs before creating the waveform generator here

There may be others, I'm a little surprised that this doesn't need to be passed to the post-processing plotting.

cc @raffi.enficiaud @mick.wright @michael.williams feel free to tag others.