Handle xforms with two arguments

Author: sebastinas

curtsies/formatstring.py

@@ -147,10 +147,9 @@ def color_str(self) -> str:
                 continue
             elif v is False:
                 continue
-            elif v is True:
+            elif k in one_arg_xforms:
                 s = one_arg_xforms[k](s)
             else:
-                # TODO: What's the purpose of this code? It will never be executed.
                 s = two_arg_xforms[k](s, v)
         return s