To not bug out on MTL files with transparency

Required to parse some MTL files with transparency variable.

Author: dav92lee

pywavefront/material.py

@@ -150,6 +150,10 @@ def parse_Ni(self, args):
         # unimplemented
         return
 
+    def parse_Tr(self, args):
+        # unimplemented
+        return
+
     def parse_illum(self, args):
         # unimplemented
         return