Updated to 0.18: music and improvements

It's a small update before I start working on more serious stuff.

Change log:

• Added some music created by Josh Culler.

• Added resolution selection to options.

• Added "invert mouse Y" option.

• Added volume sliders.

• Spell casting auto-repeats now when you hold the button.

• Moved missile starting position closer to the player to improve targetting.

• Increased lightning texture resolution.

• Improved stone cups behavior.