Allow Python interface to access IRCv3 information

[Yaulendil]

A small change to forward IRCv3 information received by Hexchat to Python scripts that request it. This addresses, though perhaps not fully, #1393.

In Python, the function passed to hexchat.hook_server_attrs() now receives the IRCv3 message as a string under the Attribute object, as Attribute.ircv3. This should not break any Python addons, unless they call dir(Attribute) and expect a very specific return value (I do not know of any that do).

It does not affect any of the other scripting interfaces because I dont know those interfaces as well.
It does not allow Python to access the IRCv3 information through hexchat.hook_print_attrs() because I wanted, at least at first, to keep my edits to the code minimal.

I have been running this fork for a week now, with an addon that heavily utilizes the change, and it seems to be quite stable.

[TingPing]

I strongly dislike just shoving the line in there and calling it a day. If we want to expose raw tags it should be in a hash table or so.

[SoniEx2]

It's a good idea to make the raw string available as well, in case anyone relies on the wire-order for some reason. Mainly nonstandard/noncompliant stuff.

[Yaulendil]

While I do see the value of the raw string being available, I do agree that it would probably be more performant to be done as a hash table in C, so that plugins dont need to spend time splitting the string up themselves.