Add conditional replace_metadata feature.

defaults.ini

@@ -161,8 +161,10 @@ extratags: FanFiction
 ## for regexp details.
 ## Make sure to keep at least one space at the start of each line and
 ## to escape % to %%, if used.
-## Two or three part lines.  Two part effect everything.
+## Two, three or five part lines.  Two part effect everything.
 ## Three part effect only those key(s) lists.
+## *Five* part lines.  Effect only when trailing conditional key=>regexp matches
+## metakey[,metakey]=>pattern=>replacement[&&metakey=>regexp]
 #replace_metadata:
 # genre,category=>Sci-Fi=>SF
 # Puella Magi Madoka Magica.* => Madoka
@@ -170,7 +172,9 @@ extratags: FanFiction
 # Crossover: (.*)=>\1
 # title=>(.*)Great(.*)=>\1Moderate\2
 # .*-Centered=>
-
+# characters=>Sam W\.=>Sam Witwicky&&category=>Transformers
+# characters=>Sam W\.=>Sam Winchester&&category=>Supernatural
+ 
 ## Some readers don't show horizontal rule (<hr />) tags correctly.
 ## This replaces them all with a centered '* * *'.  (Note centering
 ## doesn't work on some devices either.)