Add hooks to be used by users' modules

Add hooks like before_kara before the kara is loaded to be played. The user can specify actions to be done by using a custom module.

Hooks are called by commands, e.g. from functions only from the commands.h file (because it's easier and more maintenable).

Actual hooks are the following:

• kara_load when a kara is loaded (on commands play, playid, next and previous)
• queue_begin when lektord is stopped and will load a kara (on commands play and playid)