DefaultRetryMode never assigned in DiscordRestAPIClient

Issues/427 + AliasAttribute targets classes

Fixed UserTypeReader from throwing NullReferenceException

Typically, you will want to mark his method as `async`, although it is 
not required. 

Changed `his` to `this`

Add mentionable property to ModifyGuildRoleParams

Allows roles to be configured to be mentionable, fixes #424

Revert "Implement configurable command node separators"

Implement configurable command node separators

Instead of using fields, we will now use properties (that must have a setter).

This allows users to flag a field with InjectAttribute, and when the module is created at runtime, this field will be filled in with the object from the dependency map.

Allow commands to use added type readers

A.K.A. "FiniteReality is a twat"

This fixes #304 over foxbot's PR for the same issue.

Parameter preconditions and typereader overriding