configitems: register the 'transplant.filter' config

configitems: register the 'transplant.filter' config

--- a/hgext/transplant.py	Wed Oct 04 11:58:00 2017 -0400
+++ b/hgext/transplant.py	Fri Jun 30 03:44:29 2017 +0200
@@ -49,6 +49,13 @@
 # leave the attribute unspecified.
 testedwith = 'ships-with-hg-core'
 
+configtable = {}
+configitem = registrar.configitem(configtable)
+
+configitem('transplant', 'filter',
+    default=None,
+)
+
 class transplantentry(object):
     def __init__(self, lnode, rnode):
         self.lnode = lnode