integration of RegisterMessageReceiveFilter work

[bnfinet]

A while back I added the feature RegisterMessageReceiveFilter to my fork...
https://github.com/bnfinet/go-chat-bot/blob/master/cmd.go#L282-L296

This filter operation is conducted before command interpretation and execution. This allows for plugins to be created with a generalized allow/disallow and RBAC of commands based on proto/server/channel/user criteria.

Instead of opening a PR I decided to start with an issue to see if this is worthy of inclusion. If so, I'll start from the current master in this repo and (non git) cherry pick the changes, since this work is mostly from a few years back.

Thanks for your consideration.

5271579

[fabioxgn]

Hi @bnfinet this change looks good to me, if you'd like to send a PR, please do so and I'll merge it. Thanks.