Update trigger() method to support args, kwargs

Currently, you may pass arguments into callback functions like so:

```
fsm.panic(msg='killer bees')
fsm.calm('bob', msg='sedatives in the honey pots')
```

Now you can use this same syntax with the `trigger()` method:

```
fsm.trigger('panic', msg='killer bees')
fsm.trigger('calm', 'bob', msg='sedatives in the honey pots')
```