Add some IRCv3 features

[timo]

IRCv3 Features implemented:

• capability negotiation, some default requests
• tags on messages, like server time, message id, user account
• explicit batches such as multiline, chathistory or event-playback
• echo-message so we get msgids for our own outgoing messages
• requesting the latest N entries from the chathistory as events

these features of course depend on whether the server you're connected to supports the corresponding capabilities

There's some outstanding tasks:

• something to help plugins differentiate echo-messages from normal messages
  • and to make it easy for a plugin to get msgids for messages it has sent
• plugins that don't handle multiline batches could just get the text as individual messages, or as a message with unusually long text
  • same for netsplit / netjoin batches of join and quit messages
• send privmsg or notice with arbitrary tags
• send tagmsg messages
• send batches

And some other stuff that should maybe be implemented but isn't specific to IRCv3:

• parsing ISUPPORT tokens to figure things out like
  • what user modes are there
  • what channel types exist
  • maximum length of nicknames, channels, and topics
• probably some other stuff