Clear specific logs when using `rake log:clear`

There are times when I want to clear only one of the logs or just two of the logs rather than all of the logs. You can now accomplish this with:

```
$ LOGS=test rake log:clear
```
Or multiple:
````
$ LOGS=test,staging rake log:clear
```

Wasn't sure the best way to test this. I couldn't find any tests for it, so if there are any suggestions, I am more than willing to add them or provide a sample app with this successfully running in it.