Optionally pass after and before as arguments. Fixes #8.

Allows to pass `after` and `before` as task arguments, e.g.:

```shell
grunt changelog:sample:2013-03-01
grunt changelog:sample:2013-03-01:2013-04-01
```

Will come in handy once PR #7 is merged in. E.g.

```shell
grunt changelog:sample:0.3.7
```