How to configure alias options?

[ammario]

Re the discussion here, it's common to want multiple envs/flags to configure the same underlying option value esp. when refactoring the command line interface.

@mafredri proposes the concept of an OptionAlias that embeds within each Option like so:

type OptionAlias struct {
	Flag          string `json:"flag,omitempty"`
	FlagShorthand string `json:"flag_shorthand,omitempty"`
	Env           string `json:"env,omitempty"`
	Deprecated	  string `json:"deprecated,omitempty"`
}

// Option is a configuration option for a CLI application.
type Option struct {
	Aliases []OptionAlias `json:"aliases,omitempty"`
	// ...
}

// Usage:
{
	// ...
	Env: "CODER_FOOBAR",
	Alias: []serpent.OptionAlias{
		{Env: "FOOBAR", Deprecated: "Use CODER_FOOBAR instead."},
	},
},

I think the extra level of nesting on all options is unjustified when in the vast majority of options there is only one "alias".