multiline: Specify behavior with respect to formatting.

[progval]

This is the only way to format the fallback for multiline-unaware clients the same way as for multiline-aware clients

[slingamn]

lgtm

[jwheare]

I'm not sure. This puts a significant burden on clients when interpreting user input and composing a multiline message.

[progval]

So currently, if someone sends this:

BATCH +123 draft/multiline #channel
@batch=123 PRIVMSG #channel :not-bold <0x02>bold
@batch=123 PRIVMSG #channel :unclear
BATCH -123

then irccloud and the current weechat master will display it like this, because they don't implement this PR:

not-bold bold
unclear

and assuming servers don't do any reformatting, then every client not supporting multiline will get this:

PRIVMSG #channel :not-bold <0x02>bold
PRIVMSG #channel :unclear

so they will display it like this:

not-bold bold
unclear

I understand this can be annoying to implement, but we need to either:

1. accept this solution
2. propose a different solution
3. decide it's not a problem

Could we resolve this before weechat ships version 4.0 with draft/multiline support?