ircevent: allow message truncation by default

[slingamn]

After the dust settled with the truncation changes (#45, #48), we made LineBytesStrict return ErrorBodyTooLong if the message is truncated.

ircevent does not check for this error, so truncation causes outgoing messages to be dropped. This is probably not the behavior bot authors expect. We should add a config variable like DisallowTruncation (i.e., truncation is allowed by default), and then check it and check that error value.

[slingamn]

We should also expose a normalizer: thoj/go-ircevent#127

[slingamn]

I went with AllowTruncation (i.e., truncation disabled by default).