Allow conditionally required members

[swagggpickle]

This PR introduces support for variables that maybe conditionally:

var args struct {
   UserName   string `default:"abc"`
   UserID     string `default:"123"`
   Password   string `arg:"required-if:username|userid"`
}
arg.MustParse(&args)

This will enable users to express the relationship between variables. This feature is most important for featured flagged capabilities where if a feature is turned on another variable should be set in order to work correctly.

[alexflint]

Thank for this PR @swagggpickle. However, I think this is probably too niche to justify adding it to the library. I believe it is possible to achieve the same using custom validation - would you agree?

[swagggpickle]

Hi @alexflint, you are correct, it is possible to achieve the same behavior by using custom validation. I guess I was coming from the perspective that once MustParse() or Parse() has returned I would have liked to have as much validation done and not have to worry about whether or not all the variables have been set for specific feature or sub-command. Basically it would allow the user of this library to focus more on scrutinizing the values of the user input and allow the relationships between the variables to be defined in a more human readable fashion.

[alexflint]

@swagggpickle Yes there are definitely some nice things about having everything be done inside the call to Parse(). But there are costs to adding to the API, too, and in this case I don't think this change will be helpful enough to justify adding it to the API.

[swagggpickle]

That is fair enough I will cose this PR then.