aC23,
That request has been made several times in the past and is very old. One of the reasons, back then, was that there is no easy way for Plus!, and thus for a plugin, to know when something is send from your contact only. Also, Plus! only triggers events for plugins when a specific plugin command is send, not when random text is send to you.
Now, this said, Plus! has evolved since then and there are things like triggers build in which react on text in the conversation (eg: the word filters), but still no events are fired for plugins for these non-plugin commands.
I mention plugins here because these are the things which would be the (almost) first choice to make something for this; not Plus! itself.
Why not Plus! itself? Because this feature is much more complicated and involves much to many variables and settings which you can make to be included in Plus!. eg: the amount of features currently in Plus! aren't even close to what you can do which such new feature (and you don't have to make this new feature even complicated for that)...
Because this feature you request is actually what people call BOTS... They react on things you send. And as it happens, there are many bot frameworks already available. eg: "
Bot Developers Kit available! (Update 16-05)"
And if that doesn't suite you, you could always make your own bot/plugin for this.
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Thus, after said all this, I highly doubt something like that will be added in Plus!; it would be too much and indeed to bloated to put this directly into Plus!....
/ 
