Similarthreads plug-in.

Patches showthread so that the 'similar threads' table is followed by a list of similar
entries in LDM.  Similarity is determined in the same way as the vb fulltext method, using
the entry name and description as match text.

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
