Option for using underscores instead of spaces

Headphones works great, but when it renames and moving movies the files it uses spaces. Although this works, I needed underscores and not spaces. This PR adds an option in the config > Advanced Settings tab 'Use underscores instead of spaces'. It is located under the 'Renaming Options' at the bottom. Comments are welcome. 

Screenshot :
![preview thumb](http://i.imgur.com/Njqryzz.png)