Add support for hiding/showing mnemonics outside lesson/review

Cleanup and refactoring still needed.
Refactor code
Run the init() when a lookup page is loaded.
Only run the script for items that has been unlocked.

For locked items it's not possible to add own mnemonics or notes
so it's not useful to be able to hide these mnemonics.
Merge pull request #10 from nibarius/lookup-support

Lookup support