Create a `go generate` command for generating/updating flag fields from comments

[arikkfir]

It would be great if for a given struct field we could add comments for the different flag values like name, environment variable, description, etc - and have go generate create/update the corresponding Go struct tags for that field.

Example:

type MyConfig struct {
  // flag:description Everything you write from here until the
  //                  end of the comment group, including new lines
  //                  will be considered the flag's description.
  // flag:name        some-field
  // flag:env         HTTP_DEFAULT_RESPONSE_CODE
  // flag:value-name  CODE
  // flag:inherited
  ResponseCode int

  // flag:name        target
  // flag:env         HTTP_TARGET
  // flag:value-name  URL
  // flag:required
  TargetURL string

  // flag:args
  Args []string
}

This would result in the following struct:

type MyConfig struct {
  // flag:description Everything you write from here until the
  //                  end of the comment group, including new lines
  //                  will be considered the flag's description.
  // flag:name        some-field
  // flag:env         HTTP_DEFAULT_RESPONSE_CODE
  // flag:value-name  CODE
  // flag:inherited
  ResponseCode int `name:"some-field" env:"HTTP_DEFAULT_RESPONSE_CODE" valueName:"CODE" inherited:"true"`

  // flag:name        target
  // flag:env         HTTP_TARGET
  // flag:value-name  URL
  // flag:required
  TargetURL string `name:"target" env:"HTTP_TARGET" valueName:"URL" required:"true"`

  // flag:args
  Args []string `args:"true"`
}

The code generator would be idempotent, such that running it repeatedly would update the same tag and result in the same code, if the comments were unchanged.