Feature request: Allow using lambdas in postfix

[Bladieblah]

• I have marked all applicable categories:
  • documentation request (i.e. "X is missing from the documentation." If instead I want to ask "how to use X?" I understand StackOverflow#tqdm is more appropriate)
  • new feature request
• I have visited the source website, and in particular
  read the known issues
• I have searched through the issue tracker for duplicates
• I have mentioned version numbers, operating system and
  environment, where applicable:

  import tqdm, sys
  print(tqdm.__version__, sys.version, sys.platform)

I would like to use lambda functions in postfixes that automatically update attributes, which would give cleaner loops. Instead of

foo = 0
pbar = trange(50, postfix={"foo": foo})
for i in pbar:
    foo = i
    pbar.set_postfix({"foo": foo})

you could simply write

foo = 0
pbar = trange(50, postfix={"foo": lambda: foo})
for i in pbar:
    foo = i

leading to a cleaner inner loop.