Option for using underscores instead of spaces

Headphones works great, but when it renames and moves 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. 
Edit: This way it becomes Artist_Name/Artist_Name-Year-Album_Title/Artist_Name-tracknr-Title_of_the_song. Before the _ would have been spaces.

Screenshot :
![preview thumb](http://i.imgur.com/Njqryzz.png)