Add support for simplified actor data in rolls

[StasTserk]

Adds support for basic actor data syntax in various item rolls.

• Adds stat mods like @str, @con, etc.
• Adds skill modifiers like @exert @administer, etc.
• Adds level with @level
• Adds attack bonus with @atk
• Monsters try to use their hit die field as a replacement for level.

Resolves #53