Documentation for resolve parameter on prompt_choices

[hugo-cisneiros-movile]

Hi, I tried to use prompt_choices and it wasn't working with case sensitive options. For example:

Which cluster? - (Cluster1): Cluster1
Which cluster? - (Cluster1): cluster1
Which cluster? - (Cluster1): 
...

Looking at the cli.py code, there is a resolve parameter that defaults to ascii_lowercase. Later in the method, the option is passed by this resolve function and every options turns to lowercase before comparing.

I fixed this calling prompt_choices with resolve=str instead of ascii_lowercase.

It would be good if this parameter was listed/described in the API documentation :)

Thanks!

[jeffwidman]

PR welcome