This commit adds an Unmarshal method into CommandInteractionOptions. It
is probably the first commit to break the rule of keeping package
discord simple.
The goal of this rationale is that package discord should continue
prioritizing convenience and ease of use by providing small, helpful
additions without being oversized.
This method, while it uses reflect, is actually fairly small in
implementation. Its functionality should be kept to a bare minimum and
as such will not cover every single use case (but should cover most).
