Update `Trigger` to support the `inverted` kwarg

[philipstarkey]

It looks like Trigger was written without considering the DigitalOut inverted kwarg. This potentially leads to some confusing behaviour, see #93.

We should look at fixing this so it is consistent. However, case needs to be taken to ensure it is backwards compatible, particularly with PulseBlaster's as secondary pseudoclocks (as PulseBlasters have inverted digital triggers and Trigger was likely written specifically for that use case).