Cli: PersistentFlags support

[nkprince007]

Something similar to cobra's PersistentFlag would be a nice addition to save configuration to files.

[asahasrabuddhe]

@urfave/cli

I think we already can achieve the PersistentFlag functionality through the global lookup functions in the cli context. Does any of you have any other use case in mind or can we close this?

[lynncyrin]

I don't quite grasp the use case, so I'm fine closing this until someone explicitly requests it again.

[AudriusButkevicius]

I guess this is saving a flag that was set in the previous invocation of the cli

[lynncyrin]

I guess this is saving a flag that was set in the previous invocation of the cli

I believe this is what they're asking for 👍

This seems like a nice feature, but given the age of this issue i'd like to close it until someone requests it again.

[AudriusButkevicius]

I don't think this should be a problem of this library. Users can implement their own persistence.