Complex permission hierarchy

[atbrandt]

The bot should be able to support a complex owner-defined set of permissions on a per-guild basis, which determine whether or not a given user is able to invoke a given command. The permission hierarchy should support enabling or disabling commands at the following levels:

• Guild
• Channel
• Role
• Member

Hierarchy will take priority based on specificity, which is to say that if a command is disabled at the guild-level but enabled at the role level, the users with that role will still be able to execute the command. This mirrors the internal Discord permission system.

Permissions should be configurable at both the individual command level as well as at the Cog level, enabling either entire groups of commands to be affected or just individual commands. Again, specificity should matter, so that enabling an individual command when a Cog is disabled will allow that individual command to be executed. This should not affect whether or not a Cog is loaded or unloaded from the bot.