Reference time for relative plots

This adds a reference time parameter (`reftime`) to `Stream.plot` for `type='relative'`. In this way, the relative plot does not necessarily start at 0, but is in reference to some other time, such as the time of an earthquake in a simulation, or similar.

I also corrected the `Stream.plot` documentation on defaults for parameters.

There don't seem to be any tests for relative plots, so I'm not sure where to add one (if necessary).