Allow setting config path in configure call

This way when configuring programmatically (like in the example [here](http://sentry.readthedocs.org/en/latest/faq/index.html#how-do-i)):

```python
from sentry.utils.runner import configure
configure()
```

You can set the config file path:
```python
from sentry.utils.runner import configure
configure('path/to/config')
```