Support default values for short and long parameters

It's a very minor nitpick, but I found myself trying to communicate to the user the default values of different parameters in their descriptions, and then reusing those same values when parsing:

```zig
const PARAMS = clap.parseParamsComptime(
\\-o, --opt Optional argument (default: 3)
\\
);

pub fn main() !void {
...
const opt_arg = cli.args.opt orelse 3;
...
}
```

It's not very error-prone as is, since default values don't usually change much, but it would be nicer to have a way of specifying default values and having them parsed automatically.