Encapsulate more than just PRIVMSG about otrv4-over-irc

[SoniEx2]

One option would be to use a TLV for NOTICEs and TAGMSGs and another TLV for client-only tags. This seems reasonable, and send some text like "Your OTR plugin does not support this message. Please update your OTR plugin." in the main stream when the NOTICE/TAGMSG TLV is used.

So these:

@+example-client-tag=example-value PRIVMSG foo :some text
@+example-client-tag=example-value NOTICE foo :some other text
@+example-client-tag=example-value TAGMSG foo

would become something like this when OTR-encrypted:

{"message": "some text", "TLVs": {"message-tags": "+example-client-tag=example-value"}}

{"message": "Your OTR plugin does not support this message. Please update your OTR plugin.", "TLVs": {"message-ext": "NOTICE some other text", "message-tags": "+example-client-tag=example-value"}}

{"message": "Your OTR plugin does not support this message. Please update your OTR plugin.", "TLVs": {"message-ext": "TAGMSG", "message-tags": "+example-client-tag=example-value"}}