default values should be visible in help screen

As per #114 the default value for an argument should be visible in the help screen.

Some considerations:
- If the default value is a simple type and not too long in length, we can just show it.
- If it doesn't serialize well (either because there's no good way to serialize it, or because its serialization is too long to show comfortably), then we need to determine a clean alternative to showing the default. Using the type may be one option for this alternative.