Add type-set default value to help text

[dwincort]

As mentioned in #61, it would be nice if default values showed up in help messages. This PR implements such a change that I'm using personally and thought might be desired upstream.

This is a small change so that a field with <!> "defaultValue" will show (Default: defaultValue) in the help string. I opted to put this in the parseFields definition, which means that the order of field decorators now matters: that is, if one writes ... <!> "1" <?> "Field help", then the help text will read Field help (Default: 1), but if the order is ... <?> "Field help" <!> "1", then the help text will read Field help. This is by design so that the behavior could be overridden if desired.

[Gabriella439]

I think the more idiomatic way to do this is by setting the showDefault option:

https://hackage.haskell.org/package/optparse-applicative-0.16.1.0/docs/Options-Applicative.html#v:showDefault

[dwincort]

Ahh, I didn't know about that feature of optparse-applicative. Is there a straightforward way to set that from the optparse-generic interface?

[Gabriella439]

Oh, I mean that we can fix optparse-generic to automatically set showDefault on behalf of the user. I think we just forgot to set it as an oversight when adding support for default values

[dwincort]

Yeah, this is simpler and (I think) more idiomatic.

[Gabriella439]

Thank you again for contributing this! 🙂