Add IgnorePreviousMessages option to imjournal.

This option specifies whether imjournal should ignore messages
that are currently in journal. This option is only used when there
is no StateFile to avoid message loss.

	modified:   doc/imjournal.html
	modified:   plugins/imjournal/imjournal.c

Conflicts:
	doc/imjournal.html
	plugins/imjournal/imjournal.c
doc cleanup