Resolves #253

Implement Command Aliases

This implementation could probably be more efficient, but I
opted for clarity and simplicity where possible.

Fixed NullRef

Add Prefix property to Module

Add NameAttribute for overriding Name in commands/modules

Command tweaks

Preconditions for commands

The new RequireContextAttribute works just like RequireDM/RequireGuild,
but is more powerful as developers can specify multiple 'contexts' for
their command to require using the ContextType flags.

After a small discussion with Joe4evr on discord, a way of retrieving the
state of a module appeared to be needed. The new override should provide
enough context to a bot dev to allow them to do what they want.

Preconditions are still handled by CommandService.Execute