Additional config options

Hi,
Thanks for an amazing add-on. I used to have a manually rigged setup to do this in Anki 1.x, using Anki's latex support, but this is so much nicer. 

I really wanted a couple of tweaks to streamline my workflow, and these I've added as config options to the add-on. I also hit upon the issue neotek logged in your repo, that should be fixed as well.  

Please have a look. Let me know if you spot any issues, functional or otherwise, with my change. I am a beginner at Python and had no exposure to Anki/Qt/git/github so you may well find something that needs improving.
Cheers
nagul

* Added option to turn off centering of code fragments
* Added option to store the default language per deck
* Auto-update add-on config options on add-on upgrade: copy defaults for new options, delete options no longer used, leave everything else untouched
* Fix: Force save of collection when add-on config is updated - fixes issue where config changes weren't saved unless an unrelated change caused the collection state to be written to db.