Add em dash as alternative to decrease karma

As discussed in https://github.com/github/hubot-scripts/pull/1377.

---
>Some clients like [Slack](https://slack.com/) replace the `--` in
```
wellle-- for writing hubot scripts
```
to an Em dash `—`
```
wellle— for writing hubot scripts
```
which doesn't get recognized in Hubot's karma script.

>This pull requests decreases karma in the example above by triggering `karma.decrement` on Em dashes too.