Fix formatters being ignored in channel events.

Formatters need to be passed as StringBuffers to actions first.
This allows actions/plugins to change the formatters, e.g. changing
to a highlight formatter.

Change-Id: I98e55a2e2d88d423fd49d33508a225faa91a61b2
Fixes-Issue: CLIENT-486
Reviewed-on: http://gerrit.dmdirc.com/3377
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>