Regression.

This used to allow me to do `diamond -r /path/to/collector.py` and avoid having diamond loading the whole collector tree.

While I wholeheartedly support the new functionality, with this line, if I run the old format, I end up with the following values:

`diamond -r /usr/diamond/collectors/files.py`

```
collectors == {
    "FilesCollector": <class files.FilesCollector>
}

file == "/usr/diamond/collectors/files.py"
```

so `item.lower() in file.lower()` evaluates to `False`, and my collector is not run.

The filter could probably be set up after lines 356 and 358 instead of just passing `file`