Allow sorting in saved journal file(s)

[micahellison]

Use Case/Motivation

@xeruf mentioned wanting to sort the entries in the file storage itself in #821. I'm spinning it off to this issue since it's a separate concern, but I think it's worth looking into.

Example Usage

Perhaps a config value could define how it's sorted. Or, in the longer run, perhaps the sort order would be a metadata property of the journal itself, so there would be less risk of separating the stored data from its data definition.

Other Information

We're talking a lot about changing the storage format in future versions. I think issues like this can help inform those conversations, even if it we don't implement this until much later.