Add bang methods for mark_as aliases

Sometimes, I find it more clear to use the `mark_as` method instead of the infinitive. Take `notified_at` for example:

```ruby
alert = Alert.new
alert.notify!
```

It's not really clear that the method is coming from Microscope. I would much rather do this:

```ruby
alert = Alert.new
alert.mark_as_notified!
```