[io_mesh_qfmdl] Update plist parsing for `

This gets it pretty much in line with the C implementation. Certainly
good enough for validating a render pass spec :)

tools/io_mesh_qfmdl/qfplist.py

@@ -20,7 +20,7 @@
 # <pep8 compliant>
 
 quotables = ("0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
-             + "abcdefghijklmnopqrstuvwxyz!#$%&*+-./:?@|~_^")
+             + "`abcdefghijklmnopqrstuvwxyz!#$%&*+-./:?@|~_^")
 
 class PListError(Exception):
     def __init__(self, line, message):