More flexible interface for magic item bonuses

[canismarko]

Follow up to #101

Need a way to specify:

• Saving throw bonuses on a per-ability basis (7b5aac9)
• Spellcasting DC bonus
• Spellcasting attack bonus

Possible ways to do this:

• Create a Character._magic_item_bonuses method that parses the magic items and figures out the bonuses.
• Make properties (or mabye even methods) on the MagicItem class for each of the above properties.
• Something else?