support required/optional flags

[LoveFunDev]

Currently, grumble does not have "required/optional" concept for flags. Hope to have this functionality, so can define required flags and optional flags.

For example, if defined flagA as required flag, the command without flagA is invalid.
if defined flagB as optional flag, the command without flagB is valid, and the value of flagB is the default value.

[skaldesh]

Grumble at least has this concept for arguments (imho, flags should always be optional, and args either required or optional).

You can register args, which are required by default. Use grumble.Default() to make it optional. Note, that after an optional argument, only other optional arguments may follow.