Support default values for args

[smores56]

Support default values for options and parameters via a default ? [NoDefault, Value a, Generate ({} -> a)] field for single value fields (e.g. str, dec, etc.).

The thing I wanted to add but couldn't find a good way to do it was help text support. I could require the default type to implement Inspect, but that wouldn't even work for the Generate ({} -> a) variant of default since it's not known ahead of time. So for now the expectation is that the arg's help text is given a default: xyz suffix.

@isaacvando does this look good to you? I can make a release if so.

[isaacvando]

@smores56 This looks great! Thanks for adding it :)

For the help text, one idea is to require an explicit string to be added to the default tag like this:

{ default: Value 100 "100"}
# or
{ default: Value {val: 100, help: "100"}}

That being said, this idea is clunky and doesn't add much value so I think it would be better to leave it as it is now.

[smores56]

Yeah, I tried the same thing more or less and reached the same conclusion, it's much cleaner to just leave it to the developer to write their own docs