Purge historical check states cli

```
$ bin/flapjack purge check_history --days 1
Purged 2 historical check states over 1 checks.
```

and specific to a named check:

```
$ bin/flapjack purge check_history --days 0 --check jklfjdksl:567567
Nothing to do

$ bin/flapjack purge check_history --days 0 --check foo-app-01:HTTP
Purged 2 historical check states over 1 checks.
```
For consistency with `Entity.all` (and the sandstorm branch's `Check.all`) it should probably return objects.