add matchedtracks plugin

contrib/plugins/matchedtracks.py

@@ -0,0 +1,17 @@
+PLUGIN_NAME = 'Number of matched tracks'
+PLUGIN_AUTHOR = 'Nikolai Prokoschenko'
+PLUGIN_DESCRIPTION = '''Provides a scripting function called $matchedtracks, which returns the number of tracks matched in an album. Useful for distinguishing complete and incomplete releases'''
+PLUGIN_VERSION = "0.1"
+PLUGIN_API_VERSIONS = ["0.9.0", "0.10"]
+
+
+from picard.script import register_script_function
+
+def matchedtracks(parser, arg):
+    # FIXME arg is actually not needed, why is it used by eval?
+    if parser.file:
+        if parser.file.parent:
+            return str(parser.file.parent.album.get_num_matched_tracks())
+    return "0"
+
+register_script_function(matchedtracks)