Added the ability to specify the spider txt filename within the config file

Hi,

I've updated the codebase to allow you to specify the filename of the spider.txt within the config.yaml file. This allows different multiple sites to be used by wraith at once and keep a spider file for each of these separately.

If no `spider_file` is specified then it will fallback to using `spider.txt`

And example of how to implement this is below:  

```
config.yaml

....
....

#Set the filename of the spider file to use, if not specified it will fallback to spider.txt
spider_file: adt_co_uk_spider.txt
...
```

Thanks,
James