Preserve more structure in Help

[reibitto]

This idea originated from #126

Basically, if the structure of the error is preserved, it'll be possible to write a custom render function for Help. This can have uses such as for localization and different render formats (in my particular case, "compact" mode which renders with fewer newlines).

[bkirwi]

For context, there's a little structure in Help already... the command name, usage, errors, and the main body are stored separately. There's lots of opportunity to add more structure here, though... for example, preserving more information about individual options than one big string of help output.

This is unlikely to require a big redesign or anything, and should be possible to do ~incrementally; happy to review PRs with this shape.