Add a default value for @Option when the type is an Array

[fdiaz]

There's currently no way that I've found to have a default option when an @Option is an array

@Option(parsing: .upToNextOption)
var foo: [String]

I'd like to do something like:

@Option(default: ["someValue"], parsing: .upToNextOption)
var foo: [String]

So even if this option is not set, we still get this information.

[natecook1000]

@fdiaz Would you expect the default values to be included when the user specifies inputs, or would they be overridden if there's even one value given?

[fdiaz]

My thinking was that they would be indeed overwritten. I think this would be similar to how the defaults work for other types (e.g. for a String we overwrite when we pass an explicit value)