[feature request] support multiple dialects for boolean parameters

[iRyoka]

Consider the following code:

import fire

def a(flag: bool):
   if flag: 
      print("hooray!")
   else:
      print("Wanna see a flag :(")

if __name__ == '__main__': fire.Fire(a)

Now, the following options work as expected (print 'hooray!'):
python a.py --flag
python a.py --flag=True
python a.py --flag=true

However, in order to disable flag explicity the only working option is:
python a.py --flag=False
and the seemingly rational version
python a.py --flag=false
result in 'hooray' being printed as flag is parsed as a string 'false' which then casts to boolean True.

My request:
support 0, 'false' and 'no' as valid boolean arguments that all cast to False. Make this behavior optional. One way to do this is by adding a custom type to lable such flags with.

[pranshugupta01]

Hi , can i work on this issue by adding a custom function for detecting different boolean types.

[dbieber]

Another thing that works is --noflag

[dbieber]

Implementing this request would have to come as part of a larger change: using type annotations to infer types and parse arguments accordingly; this isn't planned at this time.