Add default value support

[seeruk]

This should be added to both the parameters.Argument and parameters.Option types. This will allow for further automated help-text. The default values will likely be strings, just like regular user input. Unfortunately, that does mean a runtime check will be used to validate it.

[seeruk]

Maybe we actually just need to make values support returning their zero value somehow so that we can compare the difference between a different default value, and a zero value? This way the existing default value mechanism can be kept (which makes way more sense than forcing people to add strings to structs for any type).