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.
```
That'd be nice, but unfortunately EntityCheck#self.all returns an array of strings, not entity check objects...