Similarthreads plug-in.

Patches showthread so that the 'similar threads' table is followed by a list of equally similar 
entries in LDM.

Controlling parameters are set via the LDM admin/settings page:
- Extra active/inactive
- Number of similar entries to show in total (default 5, same as VB)
- Forums in which the plugin is active (default all of them)
- Linkbit to use

Plugin selects only those entries that are visible to the user and have been moderated
