libcommuni IRC backend: support multi-lines messages

[Niluge-KiWi]

... by splitting them into one PRIVMSG command per line.

Closes #305

[Niluge-KiWi]

Done

Split on \n, send a Message for each line.

Limitations

• I did not support message splitting for the CTCP Action as I don't know what it is and cannot test it.
• I did not try to skip empty lines, it seem to create warnings on freenode.
• I did not handle \r, as I don't encounter it in my personal use, but it could be added later (tricky part: don't send an empty message for the empty string between them in \r\n)