Add IrcMessage::tags (IRCv3.2 extension)

http://ircv3.org/specification/message-tags-3.2

TODO: QVariantMap as a type of the property might not be ideal for
C++, but on the other hand it gives us full QML compatibility.
See "Data Type Conversion Between QML and C++":
http://qt-project.org/doc/qt-5/qtqml-cppintegration-data.html