Change the auto_flush_interval parameter to have a default value #50
Description
A quite common issue when using the multiline codec plugin is that the last event never gets released, which can be fixed by setting the auto_flush_interval parameter. I think this parameter should be changed to have a default value as this is the most common desired behaviour. If this is not possible as it may make it hard to unset it, it would be good if it was called out explicitly in the documentation for the plugin.