Feature: Validate input values against criteria in parameters in tags.

[pritster5]

A way to require the cmdline arg to be one of many possible, explicit choices would be helpful for input validation (something akin to enums) e.g. the cmdline arg passed in for a certain field must be either a yes or no

[nibbleshift]

Currently you can use a bool to require true or false. I could potentially add a new struct tag that would allow you to provide field validation such as:

type MySettings struct {
   Answer string `default:"no" description:"Answer either yes or no" options:"yes|no"`
}

Internally we could validate that the Answer either contains only what is listed in options.

Is this inline with what you had in mind?